feat(sessions): get_persona_state client + persona error taxonomy (v0.12.0)

Adds the read-side half of Worldtree #204's persona-state observability
surface. Pairs with v0.11.0's AffectUpdate SSE event — together they
let a consumer hydrate a persona pane on session-open (this GET) and
keep it live as turns fire (the SSE event).

Public surface:
- `get_persona_state(client, agent_id) -> dict[str, Any]` — GET
  /agents/{agent_id}/persona_state, returns the same `snapshot` dict
  shape as AffectUpdate.snapshot
- New exception types mapped from the spec's documented 4xx error_codes:
  - `PersonaNotConfigured` (404 persona_not_configured) — agent has
    no persona surface (domari, muninn, all Tier 3 in Phase 2.0)
  - `AgentNotAvailable` (404 agent_not_available) — unknown agent_id
  - `AuthScopeDenied` (403 auth_scope_denied) — key lacks the
    requested scope (persona.read here; reusable for future scoped
    endpoints)
- Other non-2xx falls through to the existing SessionApiFailed
  precedent so novel failure modes aren't silently absorbed

Tests: 6 new cases covering happy snapshot return, each typed 4xx
sub-code, unknown 404 fall-through, and 5xx SessionApiFailed parity.

Not yet consumed: TUI persona surface (Persona TabPane / sticky
header line). UX shape pending operator direction — step 3.

tests/test_sessions.py

@@ -6,11 +6,15 @@ import respx
 
 from ratatoskr.sessions import (
     AgentInfo,
+    AgentNotAvailable,
     AgentNotFound,
+    AuthScopeDenied,
     InvalidCursor,
+    PersonaNotConfigured,
     SessionApiFailed,
     SessionPage,
     create_session,
+    get_persona_state,
     list_agents,
     list_sessions,
 )
@@ -556,3 +560,108 @@ class TestListAgents:
             with pytest.raises(SessionApiFailed) as excinfo:
                 await list_agents(client)
         assert excinfo.value.status == 401
+
+
+class TestGetPersonaState:
+    """Worldtree #204 / v0.28.0 — GET /agents/{agent_id}/persona_state.
+
+    Bootstrap read for the persona snapshot — same shape as `affect_update`'s
+    `current` snapshot. Auth via `persona.read` scope (user-tier default).
+    """
+
+    @respx.mock
+    async def test_happy_full_snapshot(self) -> None:
+        """happy_full_snapshot [happy,tracer]: 200 → snapshot dict with pad +
+        dominant_emotion + emotions_active + baseline_pad + mood_drift.
+        """
+        snapshot = {
+            "agent_id": "mimir",
+            "pad": {"pleasure": 0.52, "arousal": 0.47, "dominance": 0.50},
+            "dominant_emotion": "curiosity",
+            "emotions_active": [
+                {"type": "curiosity", "intensity": 0.6, "decay_remaining_s": 202.7}
+            ],
+            "baseline_pad": {"pleasure": 0.50, "arousal": 0.40, "dominance": 0.50},
+            "mood_drift": {"valence_delta": 0.02, "arousal_delta": 0.07},
+            "last_updated_at": "2026-05-25T22:30:18+00:00",
+        }
+        respx.get("https://w.example/agents/mimir/persona_state").mock(
+            return_value=httpx.Response(200, json=snapshot)
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            result = await get_persona_state(client, "mimir")
+        assert result == snapshot
+
+    @respx.mock
+    async def test_persona_not_configured_404(self) -> None:
+        """persona_not_configured_404 [error]: 404 with error_code
+        persona_not_configured → PersonaNotConfigured. Agent exists but has
+        no persona surface (e.g. domari, muninn, Tier 3).
+        """
+        respx.get("https://w.example/agents/domari/persona_state").mock(
+            return_value=httpx.Response(
+                404, json={"error_code": "persona_not_configured", "message": "no persona"}
+            )
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(PersonaNotConfigured) as exc_info:
+                await get_persona_state(client, "domari")
+        assert exc_info.value.agent_id == "domari"
+
+    @respx.mock
+    async def test_agent_not_available_404(self) -> None:
+        """agent_not_available_404 [error]: 404 with error_code
+        agent_not_available → AgentNotAvailable. Distinct from
+        persona_not_configured — the agent_id itself is unknown.
+        """
+        respx.get("https://w.example/agents/bogus/persona_state").mock(
+            return_value=httpx.Response(
+                404, json={"error_code": "agent_not_available", "message": "unknown agent"}
+            )
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(AgentNotAvailable) as exc_info:
+                await get_persona_state(client, "bogus")
+        assert exc_info.value.agent_id == "bogus"
+
+    @respx.mock
+    async def test_auth_scope_denied_403(self) -> None:
+        """auth_scope_denied_403 [error]: 403 with error_code auth_scope_denied
+        → AuthScopeDenied. Key lacks `persona.read` scope.
+        """
+        respx.get("https://w.example/agents/mimir/persona_state").mock(
+            return_value=httpx.Response(
+                403,
+                json={"error_code": "auth_scope_denied", "message": "missing persona.read"},
+            )
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(AuthScopeDenied) as exc_info:
+                await get_persona_state(client, "mimir")
+        assert exc_info.value.scope == "persona.read"
+
+    @respx.mock
+    async def test_404_unknown_error_code_falls_through(self) -> None:
+        """404_unknown_error_code_falls_through [adversarial]: 404 without the
+        two known error codes → SessionApiFailed (don't swallow novel failure
+        modes as something more specific than they are).
+        """
+        respx.get("https://w.example/agents/mimir/persona_state").mock(
+            return_value=httpx.Response(404, json={"error_code": "novel_404"})
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(SessionApiFailed) as exc_info:
+                await get_persona_state(client, "mimir")
+        assert exc_info.value.status == 404
+
+    @respx.mock
+    async def test_500_unexpected_status(self) -> None:
+        """500_unexpected_status [error]: 5xx → SessionApiFailed (matches the
+        list_agents / list_sessions / create_session precedent)."""
+        respx.get("https://w.example/agents/mimir/persona_state").mock(
+            return_value=httpx.Response(500, content=b"boom")
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(SessionApiFailed) as exc_info:
+                await get_persona_state(client, "mimir")
+        assert exc_info.value.status == 500