feat(#20): admin (bifrost inspection + admin-events stream) onto the wt adapter (slice-6)

Slice-6 of the worldtree-sdk cutover: migrate the two admin routes off the
hand-rolled paths onto the `ratatoskr.wt` adapter over `client.admin.*`, and delete
the retired code. Both are web-only (the coverage-map's `tui.py` rows were stale —
corrected to `web/server.py`).

Adapter (`wt.py`): `get_session_bifrost` → `client.admin.sessions.bifrost` (open-world
dict verbatim, any error → SessionApiFailed default); `stream_admin_events` →
`client.admin.stream_events`, re-wrapping the SDK's `AdminEvent` → ratatoskr's at the
boundary.

Decisions (contract § slice-6 notes):
- Admin auth moves from a per-call `Authorization` header override to the client's
  `admin_auth` (`_wt_client(admin_key=…)`, extended this slice) — the SDK's admin.*
  routes use the provider, not a header.
- `AdminEvent` re-wrap (chosen over yield-through): the SDK's `admin_id`(nan)/None-able
  `type`/`data` diverge from ratatoskr's `id`/`type`/`data` that the web filter reads;
  re-wrapping (nan→0, None→""/{}) degrades the open-world None/nan ONCE at the adapter
  and keeps the web endpoint + `_admin_event_matches_web` + the `AdminEvent` domain type
  unchanged (preserves the web surface). Rejected: yield SDK events + rewire the web
  filter (heavier churn, scattered hardening).
- Admin-stream error map: a NON-200 open raises `ApiError("admin_stream_failed")`
  (NOT `ConnectFailed`) → SseConnectFailed; `ConnectionDropped` (connect-time OR
  mid-stream/resumable-EOF) → SseConnectionDropped. The web integration test caught the
  ApiError-not-ConnectFailed gotcha the unit fake couldn't.

Web (`web/server.py`): both admin endpoints build the wt client with admin_key and call
`wt.*`; the bifrost endpoint gains ConnectFailed→502 handling (cutover foot-gun); the
admin-events endpoint closes the injected transport (INV-CUT-1), never the wt client.

Deleted the hand-rolled `sessions.get_session_bifrost` + `sse_client.stream_admin_events`
(+ orphaned httpx/httpx_sse/json/AsyncIterator imports); the ratatoskr `AdminEvent`
dataclass stays in `sse_client.py` (re-wrap target, imported by wt + web) until slice-7.
Retired `test_sse_client.py` entirely (its last test was the admin stream) and the
`test_sessions.py` `TestGetSessionBifrost`; added the slice-6 adapter tests.

LIVE SMOKE (:8081, readonly-admin key) — INV-CUT-5 / DEC-4 cleared: the web bifrost
endpoint returned an admin-authed clean 404 envelope (auth + route + mapping proven);
a real `session.created` admin event (id=32) re-wrapped cleanly on live wire (driven by
a session-create, throwaway session cleaned up).

Suite 490 green; ruff clean; mypy net-improved on web/server.py (16→12 pre-existing, no
new). Patch bump 0.21.18 → 0.21.19 (the cutover MINOR is deferred to slice-7, DEC-6).

docs/coverage-map.md

@@ -97,8 +97,8 @@ sub-gap).
 | `GET /me` | ✅ | `wt.py` `get_me` (SDK `me.get`) → `cli.py` `--whoami` | **wt-adapter re-anchored (slice-5, #20)** — open-world identity dict verbatim; any error→SessionApiFailed default (401 on a bad/absent key), transport→ConnectFailed→exit 21. **LIVE-SMOKE 2026-07-19** on personal :8081 (b128): identity rendered (user_id ratatoskr, tier user, scopes incl. `character.*`, key_id c990f0be) |
 | `GET /capabilities` | ✅ | `wt.py` `get_capabilities` (SDK `capabilities.get`) → `cli.py` `--whoami` | **wt-adapter re-anchored (slice-5, #20)** — open-world advertisement verbatim; `_format_whoami` reads `allowed_roles`/`default_role` and degrades on a null/non-mapping template (slice-4 hardening); matches conversation-api-spec **v1.1** (`b4a278c`). **LIVE-SMOKE 2026-07-19**: `ephemeral_template echo: default=echo max_bytes=32768 roles=[echo]` |
 | `GET /sessions/{id}/tools` | ✅ | `sessions.py:411` `get_session_tools` → `tui.py` `_hydrate_session_tools` | owner-scoped tool inventory in the TUI Tools pane (#183) |
-| `GET /admin/sessions/{id}/bifrost` | ✅ | `sessions.py:428` `get_session_bifrost` → `tui.py` `_hydrate_bifrost_state` | admin-scoped BifrostState pane (#176); admin key (`RATATOSKR_ADMIN_API_KEY`); live-auth-proven |
-| `GET /admin/events` (SSE) | ✅ | `sse_client.py` `stream_admin_events` → `tui.py` `_stream_admin_events` | admin lifecycle SSE stream (#11), session-filtered AdminEvents pane; admin key; live-auth-proven |
+| `GET /admin/sessions/{id}/bifrost` | ✅ | `wt.py` `get_session_bifrost` (SDK `admin.sessions.bifrost`) → `web/server.py` `_session_bifrost_endpoint` | **wt-adapter re-anchored (slice-6, #20)** — admin-scoped BifrostState (#176); admin_auth rides on the wt client (`_wt_client(admin_key=…)`), NOT a per-call header; open-world dict verbatim, any error→SessionApiFailed default. **LIVE-SMOKE 2026-07-19** on :8081 (readonly-admin key): admin-authed end-to-end (404 `session_not_bifrost_bound` clean envelope — auth + route + mapping proven). (Consumer is `web/server.py`, not `tui.py` — the old row was stale.) |
+| `GET /admin/events` (SSE) | ✅ | `wt.py` `stream_admin_events` (SDK `admin.stream_events`) → `web/server.py` `_admin_events_endpoint` | **wt-adapter re-anchored (slice-6, #20)** — admin lifecycle SSE (#11), session-filtered; admin_auth on the wt client; the adapter re-wraps the SDK's `AdminEvent`→ratatoskr's (nan `admin_id`→id 0, None type/data→`""`/`{}`), non-200 open `ApiError`→SseConnectFailed, `ConnectionDropped`→SseConnectionDropped. **LIVE-SMOKE 2026-07-19**: a real `session.created` event (id=32) re-wrapped cleanly on live wire. (Consumer is `web/server.py`, not `tui.py` — stale row corrected.) |
 | `GET /models/available-for-characters` | ✅ | `wt.py` `list_character_models` (SDK `models.available_for_characters`) → `cli.py` `--characters` | **wt-adapter re-anchored (slice-5, #20)** — open-world catalog verbatim; the probe reads `items` null-safe (`or []`); any error→SessionApiFailed default. **LIVE-SMOKE 2026-07-19**: `character models: char-rp` |
 | `POST /characters` | ✅ | `wt.py` `create_character` (SDK `characters.create`) → `cli.py` `--characters` | **wt-adapter re-anchored (slice-5, #20)** — body `{character}` (+`state` only when set — SDK-idiomatic, drops the redundant explicit null); open-world create ACK verbatim; the probe degrades on a missing `character_id` (no hard-index). **LIVE-SMOKE 2026-07-19**: `created char_8c00006e…` |
 | `GET /characters/{id}/state` | ✅ | `wt.py` `get_character_state` (SDK `characters.state`) → `cli.py` `--characters` | **wt-adapter re-anchored (slice-5, #20)** — open-world live PAD/emotions verbatim; TTL-refreshing read. **LIVE-SMOKE 2026-07-19**: `state pad=[0.234, -0.136, 0.065]` read back |