feat(#2): BifrostState pane — GET /admin/sessions/{id}/bifrost (admin-key)

v1 coverage-audit: the last unbuilt design-brief §5 debug widget. First
admin-key consumer in ratatoskr.

- sessions.py: get_session_bifrost(client, session_id, *, admin_key) —
  admin-scoped (admin.sessions.read); the request overrides Authorization
  with admin_key (distinct from the consumer bearer). 200 -> dict, non-200
  -> SessionApiFailed (403 scope-denied, 404 not-bound).
- cli.py: --admin-key flag + RATATOSKR_ADMIN_API_KEY env -> ParsedArgs.admin_key.
- tui.py: new "Bifrost" TabPane + _format_bifrost_state + _hydrate_bifrost_state
  best-effort worker (unconditional on_mount). Writes {endpoint, connected,
  caps, tools} + audits; self-labels "not configured" / "not bound" / graceful
  on 403+error, never crashes.
- Contract #2 amended (FN, incl. the bearer-override POST) + validated. TDD:
  4 wrapper tests + 1 format unit + 3 hydrate integration. Suite 552 green.
- LIVE-AUTH-PROVEN on :8081 (admin key reached resource-layer 404, not 401/403).

Ledger correction: #11 (AdminEvents) is NO LONGER BLOCKED — the admin key
was verified to carry admin.events.read; only the pane is unbuilt. Coverage:
REST 11/40.

tests/test_sessions.py

@@ -21,6 +21,7 @@ from ratatoskr.sessions import (
     get_capabilities,
     get_me,
     get_persona_state,
+    get_session_bifrost,
     get_session_tools,
     list_agents,
     list_sessions,
@@ -1039,3 +1040,65 @@ class TestGetSessionTools:
             with pytest.raises(AssertionError):
                 await get_session_tools(client, "")
         assert route.call_count == 0
+
+
+class TestGetSessionBifrost:
+    """#2 contract — get_session_bifrost (GET /admin/sessions/{id}/bifrost, #176)."""
+
+    @respx.mock
+    async def test_happy_uses_admin_bearer(self) -> None:
+        """happy [happy,tracer]: 200 → binding dict; request carries the ADMIN bearer (override)."""
+        route = respx.get("https://w.example/admin/sessions/s1/bifrost").mock(
+            return_value=httpx.Response(
+                200,
+                json={
+                    "endpoint_url": "https://bifrost.example/mcp",
+                    "consumer_id": "alice",
+                    "connected": True,
+                    "capabilities_granted": ["tools:call", "tools:read"],
+                    "tools": [{"name": "bifrost.alice.echo", "description": "echo"}],
+                },
+            )
+        )
+        async with httpx.AsyncClient(
+            base_url="https://w.example",
+            headers={"Authorization": "Bearer consumer-key"},
+        ) as client:
+            state = await get_session_bifrost(client, "s1", admin_key="admin-xyz")
+        assert state["connected"] is True
+        assert state["tools"][0]["name"] == "bifrost.alice.echo"
+        # the request overrode the client's default consumer bearer with the admin key
+        assert route.calls[0].request.headers["Authorization"] == "Bearer admin-xyz"
+
+    @respx.mock
+    async def test_403_scope_denied(self) -> None:
+        """403 [error]: admin key lacks admin.sessions.read → SessionApiFailed(403)."""
+        respx.get("https://w.example/admin/sessions/s1/bifrost").mock(
+            return_value=httpx.Response(403, json={"error_code": "auth_scope_denied"})
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(SessionApiFailed) as exc:
+                await get_session_bifrost(client, "s1", admin_key="k")
+        assert exc.value.status == 403
+
+    @respx.mock
+    async def test_404_not_bound(self) -> None:
+        """404 [error]: session_not_bifrost_bound → SessionApiFailed(404)."""
+        respx.get("https://w.example/admin/sessions/s1/bifrost").mock(
+            return_value=httpx.Response(404, json={"error_code": "session_not_bifrost_bound"})
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(SessionApiFailed) as exc:
+                await get_session_bifrost(client, "s1", admin_key="k")
+        assert exc.value.status == 404
+
+    @respx.mock
+    async def test_empty_admin_key_asserts(self) -> None:
+        """empty_admin_key [adversarial]: '' → AssertionError; no HTTP issued."""
+        route = respx.get("https://w.example/admin/sessions/s1/bifrost").mock(
+            return_value=httpx.Response(200, json={})
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(AssertionError):
+                await get_session_bifrost(client, "s1", admin_key="")
+        assert route.call_count == 0