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.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.11.0"
+version = "0.12.0"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"