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.
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