feat(#2): Tier-2 — transient characters + persona-state write; audit converges

v1 coverage-audit: the last in-scope client I/O points. The audit now
CONVERGES — REST 17/40 covered with zero in-scope gaps (23 excluded-by-
design), SSE 11/11, Bifrost planes 8/8.

- sessions.py: list_character_models / create_character / get_character_state
  / delete_character (#161, character.read/write) + set_persona_state
  (POST /sessions/{id}/persona_state — freeform body, unpinned in the
  frozen surface). 200/201 -> dict (or None on 204), off-status ->
  SessionApiFailed.
- cli.py: two one-shot probes (mirror --whoami): --characters (CRUD
  lifecycle report) + --set-persona-pad "p,a,d" (requires --session).
  New ParsedArgs.characters/set_persona_pad + probe mutual-exclusion.
- Contract #2 amended (5 FNs) + validated. TDD: 7 wrapper + 5 cli tests.
  Suite 573 green; touched code ruff-clean.
- Char read side live-proven (GET /models/available-for-characters -> 200).

Coverage-map: convergence frontier CLOSED — scope-A "done" (every frozen
I/O point classified) is met; ratatoskr cuts v1 when Worldtree tags 1.0.

tests/test_sessions.py

@@ -16,15 +16,20 @@ from ratatoskr.sessions import (
     PersonaNotConfigured,
     SessionApiFailed,
     SessionPage,
+    create_character,
     create_session,
+    delete_character,
     endpoint_for_plane,
     get_capabilities,
+    get_character_state,
     get_me,
     get_persona_state,
     get_session_bifrost,
     get_session_tools,
     list_agents,
+    list_character_models,
     list_sessions,
+    set_persona_state,
 )
 
 
@@ -1102,3 +1107,89 @@ class TestGetSessionBifrost:
             with pytest.raises(AssertionError):
                 await get_session_bifrost(client, "s1", admin_key="")
         assert route.call_count == 0
+
+
+class TestTransientCharacters:
+    """docs/contracts/issues/2.contract.md — transient-character wrappers (#161)."""
+
+    @respx.mock
+    async def test_list_models(self) -> None:
+        """list_models [happy,tracer]: 200 → {items:[...]} verbatim."""
+        respx.get("https://w.example/models/available-for-characters").mock(
+            return_value=httpx.Response(200, json={"items": [{"name": "fast", "thinking": False}]})
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            models = await list_character_models(client)
+        assert models["items"][0]["name"] == "fast"
+
+    @respx.mock
+    async def test_create_body_and_response(self) -> None:
+        """create [happy]: body is {character, state}; 201 → {character_id, ttl_expires_at}."""
+        import json as _json
+
+        route = respx.post("https://w.example/characters").mock(
+            return_value=httpx.Response(201, json={"character_id": "char_x", "ttl_expires_at": "t"})
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            out = await create_character(client, {"schema_version": "1", "name": "H"})
+        assert out["character_id"] == "char_x"
+        body = _json.loads(route.calls[0].request.content)
+        assert body == {"character": {"schema_version": "1", "name": "H"}, "state": None}
+
+    @respx.mock
+    async def test_get_state(self) -> None:
+        """get_state [happy]: 200 → live PAD/emotions snapshot."""
+        respx.get("https://w.example/characters/char_x/state").mock(
+            return_value=httpx.Response(200, json={"schema_version": "1", "pad": [0.4, 0.1, -0.2]})
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            state = await get_character_state(client, "char_x")
+        assert state["pad"] == [0.4, 0.1, -0.2]
+
+    @respx.mock
+    async def test_delete_204(self) -> None:
+        """delete [happy]: 204 → None."""
+        respx.delete("https://w.example/characters/char_x").mock(
+            return_value=httpx.Response(204)
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            assert await delete_character(client, "char_x") is None
+
+    @respx.mock
+    async def test_create_403_scope(self) -> None:
+        """create_403 [error]: key lacks character.write → SessionApiFailed(403)."""
+        respx.post("https://w.example/characters").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 create_character(client, {"name": "H"})
+        assert exc.value.status == 403
+
+
+class TestSetPersonaState:
+    """#2 contract — set_persona_state (POST /sessions/{id}/persona_state)."""
+
+    @respx.mock
+    async def test_happy_204(self) -> None:
+        """happy [happy,tracer]: freeform snapshot body; 204 → None."""
+        import json as _json
+
+        route = respx.post("https://w.example/sessions/s1/persona_state").mock(
+            return_value=httpx.Response(204)
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            result = await set_persona_state(client, "s1", {"pad": [0.4, 0.1, -0.2]})
+        assert result is None
+        assert _json.loads(route.calls[0].request.content) == {"pad": [0.4, 0.1, -0.2]}
+
+    @respx.mock
+    async def test_non_204_raises(self) -> None:
+        """non_204 [error]: 422 (bad snapshot shape) → SessionApiFailed(422)."""
+        respx.post("https://w.example/sessions/s1/persona_state").mock(
+            return_value=httpx.Response(422, json={"error_code": "validation_failed"})
+        )
+        async with httpx.AsyncClient(base_url="https://w.example") as client:
+            with pytest.raises(SessionApiFailed) as exc:
+                await set_persona_state(client, "s1", {"pad": [1, 2, 3]})
+        assert exc.value.status == 422