feat(#20): agents/tier3 family onto the wt adapter + model→role fold (slice-4)

Cut ratatoskr's consumer agent-lifecycle routes over to worldtree-sdk
(issue #20 slice-4). Five routes now flow through `ratatoskr.wt` over the
SDK's `client.agents.*`, returning open-world dicts and mapping the SDK's
undiscriminated `ApiError` floor by route+(status,error_code) per INV-CUT-2:

- `list_agents`      → `agents.list`
- `get_persona_state`→ `agents.persona_state` (404 persona_not_configured /
                        404 agent_not_available / 403 auth_scope_denied)
- `define_agent`     → `agents.define` (429→Tier3QuotaExceeded(retry_after=0),
                        403→Tier3UserIdUnsupported, 422 layer_deferred→…)
- `patch_agent`      → `agents.patch`  (404→Tier3AgentNotFound, 422 field_not_mutable)
- `delete_agent`     → `agents.delete` (404→Tier3AgentNotFound; NOT hide-existence)

Rewired call-sites: the `python -m ratatoskr.tier3` CLI (define/patch/delete)
and the web `_agents_endpoint` / `_persona_state_endpoint`, both catching the
SDK's `ConnectFailed` transport-failure normalization. Deleted the hand-rolled
paths: `sessions.list_agents` / `get_persona_state` / `AgentInfo`, and
`tier3.define/patch/delete_agent` / `Tier3AgentInfo` / parse+extract helpers.

model→role fold (scope B): the define/patch response echoes `role` (spec 1.2 /
b128), read off the open-world dict; `LocalAgentEntry.model`→`.role`,
local-index schema v1→2 (old index discarded, no-backwards-compat).

The Tier-3 caller-semantic exceptions move to `sessions.py`: running the CLI
as `__main__` while `wt` imports `ratatoskr.tier3` bound two copies of each
exception class, so a raised `Tier3AgentNotFound` escaped the CLI's `except`
as an uncaught traceback. Homing them in `sessions` (never `__main__`) makes
the class identity single. The live smoke — not the unit tests, which call
`main()` in-process — caught this.

Error-map rows + slice-4 notes added to the cutover contract; coverage-map
re-anchored. LIVE-SMOKE on personal :8081 (b128): define(thoughtful-character)
→ patch → list(6 agents) → persona_state(→PersonaNotConfigured mapped) →
delete → index empty; non-existent-id patch via `-m` → [agent_not_found]
exit 20. Suite 465 green.

docs/contracts/worldtree_sdk_cutover.contract.md

@@ -164,7 +164,15 @@ others, they get their own row here — the default is NOT a general "any 404
 | `ApiError(404)` on `sessions.write_history` | `AuthoredHistoryUnavailable` (hide-existence) |
 | `ApiError(422 cursor_invalid)` on `sessions.list` | `InvalidCursor` (dual-key: status 422 AND error_code; the flat cursor body surfaces the code) |
 | `ApiError(502)` on bound `sessions.create` | `BifrostHandshakeFailed` — NOT gated on error_code (unlike list's 422): INV-002, the synchronous handshake is the SOLE bound-502 cause; and the SDK's envelope parser prefers the nested `detail` (which carries `bifrost_error`, not `error_code`), so no distinguishing top-level `error_code` surfaces. The route+status IS the discriminator. |
-| **`ApiError` (any other status/route) — the default** | `SessionApiFailed(status, error_code, body)` |
+| `ApiError(429)` on `agents.define` (slice-4) | `Tier3QuotaExceeded(retry_after=0)` — the SDK's `ApiError` floor carries no response headers, so the `Retry-After` header the hand-rolled path read is unavailable; spec §2675 pins Phase-2.0 quota to `Retry-After: 0`, so the adapter defaults to 0. A non-zero forward-compat value is unrecoverable until the SDK surfaces headers (INFORM wtsdk-dev; reference-impl posture). |
+| `ApiError(403 tier3_user_id_unsupported)` on `agents.define` (slice-4) | `Tier3UserIdUnsupported` (dual-key: status 403 AND error_code) |
+| `ApiError(422 layer_deferred)` on `agents.define` (slice-4) | `Tier3LayerDeferred(field)` — `field` parsed from the body (`detail.field` / flat `field`); the SDK carries `error_code` but not `field`, so the adapter body-parses it (same posture as bound-502's `bifrost_error`) |
+| `ApiError(404)` on `agents.patch` / `agents.delete` (slice-4) | `Tier3AgentNotFound` (route-discriminated; agents CRUD is NOT a hide-existence route — a 404 there IS "no such agent") |
+| `ApiError(422 field_not_mutable)` on `agents.patch` (slice-4) | `Tier3FieldNotMutable(field)` (dual-key status+error_code; `field` body-parsed) |
+| `ApiError(404 persona_not_configured)` on `agents.persona_state` (slice-4) | `PersonaNotConfigured` (dual-key) |
+| `ApiError(404 agent_not_available)` on `agents.persona_state` (slice-4) | `AgentNotAvailable` (the persona-surface `sessions.AgentNotAvailable`, distinct from the eager-turn `sse_client.AgentNotAvailable`; dual-key) |
+| `ApiError(403 auth_scope_denied)` on `agents.persona_state` (slice-4) | `AuthScopeDenied(scope="persona.read")` (dual-key) |
+| **`ApiError` (any other status/route, incl. `agents.list` and any unmatched agent-route code) — the default** | `SessionApiFailed(status, error_code, body)` |
 
 The default row is load-bearing: any `ApiError` not matched above surfaces as the
 generic `SessionApiFailed` carrying the raw `status`/`error_code`/`body` — the
@@ -206,6 +214,32 @@ re-anchor its coverage-map rows.
    SSE parsing); retire contracts #2/#15; final coverage-map re-anchor; minor bump
    (DEC-6, operator approval).
 
+### Slice-4 notes (Agents/Tier-3 + `model`→`role` fold, decided at TDD)
+
+- **`model`→`role` cutover folds in here (scope B).** Worldtree spec 1.2 (`v1.0.0b128`,
+  live on :8080/:8081) made the `/agents/define` response echo `role`, closing the
+  old W-4 `model` echo. The adapter returns the SDK's OPEN-WORLD `DefinedAgent` /
+  `PatchedAgent` dicts verbatim (parity posture); callers read `info["role"]`. The
+  frozen `Tier3AgentInfo` dataclass (which read `body["model"]` and would KeyError
+  post-b128) is DELETED — no dataclass normalization layer survives.
+- **`ratatoskr.local_agents` schema bump.** `LocalAgentEntry.model` → `.role` (the
+  field stores what the wire now calls a role); `_SCHEMA_VERSION` 1→2 so any
+  pre-cutover on-disk index is discarded cleanly (no-backwards-compat, DEC-3).
+- **`AgentNotAvailable` name collision.** `sessions.AgentNotAvailable` (persona-state
+  404 `agent_not_available`) and `sse_client.AgentNotAvailable` (eager-turn 409) are
+  distinct types that share a name; `wt` already imports the sse_client one for the
+  stream, so it imports the persona one ALIASED (`PersonaAgentNotAvailable`) and
+  raises it from `get_persona_state`. The web endpoint keeps importing the persona
+  `AgentNotAvailable` from `sessions` (same class), so its `except` is unchanged.
+- **`ConnectFailed` at every rewired caller (slice-3 foot-gun).** The SDK normalizes
+  ANY transport failure to `ConnectFailed(status=0)` (`request.py`), not a raw httpx
+  error. The rewired tier3 CLI and both web endpoints (`_agents_endpoint`,
+  `_persona_state_endpoint`) catch `wtsdk.ConnectFailed` → their existing
+  network-error surface (CLI exit 21 / web 502). The web `test_network_error_returns_502`
+  (respx `httpx.ConnectError` side-effect) is the RED that proves this.
+- **`agents.get(agent_id)`** (SDK `GET /agents/{id}`) is NOT wrapped — ratatoskr has no
+  `get_agent` consumer; only list/persona_state/define/patch/delete are in coverage.
+
 ## Out of scope
 
 - Bifrost PROVIDER planes (memory/affect) — hand-rolled, ADR-0009, untouched.