feat(#20): persona + authored-history + first-message onto the wt adapter (slice-3)

Slice-3 of the worldtree-sdk cutover: migrate the session persona-state write,
the #347 authored-history write, and get_session_messages onto ratatoskr.wt, and
route the first-message preset seed through the adapter. Retire the last
hand-rolled sessions.py paths the --seed-first-message probe kept alive
(create_session + SessionInfo, set_persona_state, write_authored_history,
get_session_messages, _bifrost_error_from).

- wt.set_persona_state (SDK PadState) — the CLI passes three finite PAD axes; the
  SDK owns the {"pad": {...}} wire (#317). No route-specific error row → the
  SessionApiFailed default.
- wt.write_authored_history (SDK write_history) — v1 author=assistant; 404 →
  AuthoredHistoryUnavailable (hide-existence; the route is the discriminator,
  never the body); every other ApiError → the default. Drops the unused
  author/effects/claimed_original_at params (no caller uses them).
- first_message.seed_preset_first_message now takes a WorldtreeClient and routes
  through wt.write_authored_history; the best-effort invariants (INV-001..004,
  never-raise/never-block/one-write/zero-worldtree-source-import) are unchanged.
  Tests drive a fake WorldtreeClient — the wire is the SDK's to prove.
- CLI --set-persona-pad / --seed-first-message + the _amain and web create-path
  first-message seeds rewired onto the adapter. --set-persona-pad pre-validates
  PAD finiteness (clean usage_error, never a crash on the SDK ConfigurationError).

LIVE-SMOKE on personal :8081 (b128, INV-CUT-5): --seed-first-message → 201
(seq=0, phase=seeded) → read-back verbatim; --set-persona-pad → 204; the --new
create-path preset seed observed routing through the adapter. All slice-3 route
families proven end-to-end through the ratatoskr surface.

docs/coverage-map.md + first_message.contract.md re-anchored onto the adapter;
the slice-2 create/stream/cancel rows re-anchored too (they still named the
deleted sse_client/sessions symbols).

Suite 466 green; mypy no new errors (baseline 22 → 20 in the touched modules);
ruff clean. INV-CUT-1..5 held. Bifrost provider planes untouched.

docs/contracts/first_message.contract.md

@@ -13,11 +13,12 @@ scope: >
   replacement for a system-prompt "startup" instruction. Two entry points:
   `preset_for` (lookup) and `seed_preset_first_message` (best-effort seed).
   Consumed by ratatoskr.cli (the `--new` session path) and ratatoskr.web.server
-  (the POST /api/sessions endpoint). Depends on ratatoskr.sessions
-  (write_authored_history + its exceptions); no core.* / worldtree.* imports.
+  (the POST /api/sessions endpoint). Depends on ratatoskr.wt
+  (`wt.write_authored_history` + AuthoredHistoryUnavailable) over a WorldtreeClient
+  (worldtree-sdk cutover slice-3, #20); no core.* / worldtree.* SOURCE imports.
 depends_on:
-  - "httpx"
-  - "ratatoskr.sessions"
+  - "worldtree_sdk"
+  - "ratatoskr.wt"
 used_by:
   - "ratatoskr.cli"
   - "ratatoskr.web.server"
@@ -26,7 +27,7 @@ complexity: "low"
 estimated_loc: 60
 confidence: 0.9
 assumptions:
-  - "write_authored_history (contract #2 amendment 2026-07-06) is the seed primitive: 200/201 → ack dict, 404 → AuthoredHistoryUnavailable (hide-existence), other non-2xx → SessionApiFailed."
+  - "wt.write_authored_history (the SDK-adapter seed primitive, #20) writes over the worldtree-sdk client: success → ack mapping, 404 → AuthoredHistoryUnavailable (hide-existence), other ApiError → wt.SessionApiFailed. Behavior/semantics unchanged from the retired hand-rolled path — only the transport moved to the SDK."
   - "The preset registry is a static in-module dict keyed by agent_id; editing it is how an operator tunes an agent's opening. Seeded with ratatoskr:sindra only."
   - "Auto-seed is BEST-EFFORT and MUST NOT block session creation: an instance without the session.history.write grant returns the hide-404, which is swallowed (session opens with no seeded greeting)."
 ---
@@ -47,8 +48,8 @@ every new session for a preset agent opens in-character regardless of surface.
 
 ## Data flow
 
-**In:** a live `httpx.AsyncClient` (caller-owned, base_url + bearer set), a fresh
-`session_id`, and the bound `agent_id`.
+**In:** a `WorldtreeClient` (the wt-adapter client, built over ratatoskr's
+caller-owned transport), a fresh `session_id`, and the bound `agent_id`.
 
 **Out:** on a preset agent, one `POST /sessions/{session_id}/history` (author=assistant,
 the preset text, per-content idempotency key). Returns the seeded content on
@@ -104,19 +105,19 @@ TESTS:
   preset_miss [happy]: preset_for("mimir") is None
   empty_agent_id [adversarial]: preset_for("") → AssertionError
 
-FN seed_preset_first_message(client: httpx.AsyncClient, session_id: str, agent_id: str) -> str | None
-BRIEF: Best-effort seed of an agent's preset opening as a #347 authored first-message on session_id. If agent_id has a preset, POST it via write_authored_history (author=assistant, per-content idempotency key, the await bounded by asyncio.wait_for(_SEED_TIMEOUT_S)) and return the seeded content; on no-preset, a malformed input, OR ANY exception except asyncio.CancelledError, return None WITHOUT raising. Never raises (except CancelledError, which propagates) and never blocks session creation — it is wired into three create paths.
+FN seed_preset_first_message(client: WorldtreeClient, session_id: str, agent_id: str) -> str | None
+BRIEF: Best-effort seed of an agent's preset opening as a #347 authored first-message on session_id. If agent_id has a preset, write it via wt.write_authored_history (author=assistant, per-content idempotency key, the await bounded by asyncio.wait_for(_SEED_TIMEOUT_S)) and return the seeded content; on no-preset, a malformed input, OR ANY exception except asyncio.CancelledError, return None WITHOUT raising. Never raises (except CancelledError, which propagates) and never blocks session creation — it is wired into the CLI + web create paths.
 PRE: [PRE-001 hard] client is not None -- soft-guarded: return None (NOT assert) if violated, so a wiring bug can't crash the create path (INV-001)
 PRE: [PRE-002 hard] session_id is a non-empty str -- soft-guarded: return None if violated
 PRE: [PRE-003 hard] agent_id is a non-empty str -- soft-guarded: return None if violated (also guards FIRST_MESSAGE_PRESETS.get against a non-hashable/non-str id)
 POST: [POST-001 return_value] preset agent + successful write → returns the preset text; no-preset, malformed input, OR any swallowed failure → None
-POST: [POST-002 side_effect] a no-preset / malformed-input call issues ZERO HTTP; a preset agent issues exactly one POST /sessions/{session_id}/history with body author="assistant", content=preset, idempotency_key="ratatoskr-preset-"+sha256(preset)[:12], the await bounded by _SEED_TIMEOUT_S so a stalled response cannot block
+POST: [POST-002 side_effect] a no-preset / malformed-input call issues ZERO writes; a preset agent issues exactly one authored-history write (POST /sessions/{session_id}/history via the SDK) with entry author="assistant", content=preset, idempotency_key="ratatoskr-preset-"+sha256(preset)[:12], the await bounded by _SEED_TIMEOUT_S so a stalled response cannot block
 ERROR_ROUTING:
   asyncio.CancelledError:
     local_handling: RE-RAISE (cancellation is not a seed failure; never swallow it — and it is a BaseException, so `except Exception` would miss it anyway)
     flow_control: propagate
     state_recovery: n/a
-  any other Exception (hide-404 AuthoredHistoryUnavailable, SessionApiFailed 409/422/etc., httpx.HTTPError, TimeoutError from wait_for, any unexpected error):
+  any other Exception (hide-404 AuthoredHistoryUnavailable, wt.SessionApiFailed 409/422/etc., SDK ConnectFailed, TimeoutError from wait_for, any unexpected error):
     local_handling: swallow; return None
     flow_control: continue (never blocks session create)
     state_recovery: session opens with no seeded greeting
@@ -125,18 +126,18 @@ STEPS:
   2. [sequential, prescriptive] content = FIRST_MESSAGE_PRESETS.get(agent_id); IF content is None: RETURN None (INV-002 — zero HTTP)
   3. [sequential, prescriptive] Soft-guard: IF client is None OR session_id is not a non-empty str: RETURN None
   4. [sequential, prescriptive] key = "ratatoskr-preset-" + sha256(content utf-8)[:12]
-  5. [sequential, prescriptive] TRY: await asyncio.wait_for(write_authored_history(client, session_id, content=content, idempotency_key=key), timeout=_SEED_TIMEOUT_S)
+  5. [sequential, prescriptive] TRY: await asyncio.wait_for(wt.write_authored_history(client, session_id, content=content, idempotency_key=key), timeout=_SEED_TIMEOUT_S)
      tool: { destructive: false, idempotent: true, read_only: false, open_world: false }
   6. [branch, prescriptive] EXCEPT asyncio.CancelledError: RAISE; EXCEPT Exception: RETURN None
   7. [cleanup, prescriptive] RETURN content
-TESTS:
-  seeds_preset [happy,tracer]: preset agent, mock 201 → returns the preset text; exactly one POST /sessions/{id}/history; body author="assistant" + content=preset + idempotency_key="ratatoskr-preset-"+sha256(preset)[:12]
-  no_preset_zero_http [happy]: agent "mimir" → returns None; NO HTTP issued
-  feature_absent_swallowed [error]: preset agent, mock 404 session_not_found → returns None, no raise
-  session_api_failed_swallowed [error]: preset agent, mock 409 → returns None, no raise
-  transport_error_swallowed [error]: preset agent, mock httpx.ConnectError → returns None, no raise
+TESTS: (driven through a fake WorldtreeClient whose sessions.write_history returns/raises — the wire is the SDK's to prove via its parity corpus)
+  seeds_preset [happy,tracer]: preset agent, fake write_history returns an ack → returns the preset text; exactly one write_history call; entry author="assistant" + content=preset + idempotency_key="ratatoskr-preset-"+sha256(preset)[:12]
+  no_preset_zero_write [happy]: agent "mimir" → returns None; ZERO write_history call
+  feature_absent_swallowed [error]: preset agent, fake raises ApiError(404) → adapter maps to AuthoredHistoryUnavailable → returns None, no raise
+  session_api_failed_swallowed [error]: preset agent, fake raises ApiError(409) → wt.SessionApiFailed → returns None, no raise
+  transport_error_swallowed [error]: preset agent, fake raises SDK ConnectFailed → returns None, no raise
   unexpected_exception_swallowed [error]: preset agent, write raises ValueError → returns None, no raise (INV-001 broad never-raise)
   cancellation_propagates [error]: preset agent, write raises asyncio.CancelledError → RE-RAISED (never swallowed)
-  malformed_agent_id_no_http [adversarial]: agent_id=123 (non-str) OR "" → None; NO HTTP; no raise
-  empty_session_id [adversarial]: session_id="" (preset agent) → None (soft guard); NO HTTP; no raise
+  malformed_agent_id_no_write [adversarial]: agent_id=123 (non-str) OR "" → None; ZERO write; no raise
+  empty_session_id [adversarial]: session_id="" (preset agent) → None (soft guard); ZERO write; no raise
 ```