feat(#20): characters + me/capabilities/models onto the wt adapter (slice-5)

Slice-5 of the worldtree-sdk cutover: migrate the remaining consumer READS +
transient-character CRUD off the hand-rolled httpx wrappers onto the
`ratatoskr.wt` adapter over the SDK, and delete the retired path.

Adapter (`wt.py`): add `get_me` / `get_capabilities` / `list_character_models`
/ `create_character` / `get_character_state` / `delete_character` over
`client.me` / `client.capabilities` / `client.models` / `client.characters.*`.
All six are open-world reads/acks returned verbatim; none carries a
discriminated SDK error, so each maps any `ApiError` → the `SessionApiFailed`
default (INV-CUT-2) — exact parity with the retired path. No new Error-map rows.

Decisions (contract § slice-5 notes): `create_character` omits `state` when None
(SDK-idiomatic inline literal, server-equivalent to the retired explicit null);
`delete_character` returns the SDK's open ACK verbatim (`-> Mapping|None`, not
normalized to None).

CLI rewire (`cli.py`): `--whoami` (me + capabilities) and `--characters`
(models → create → state → delete) build a `wt.build_client` over the injected
probe transport and catch `wt.SessionApiFailed` + `ConnectFailed`. Open-world
degrade-not-crash carried (cumulative cutover foot-gun): `_characters_probe`
reads `items` null-safe and extracts `character_id` defensively (clean abort,
no hard-index KeyError); `_format_whoami` widened to `Mapping`.

Deleted the six hand-rolled `sessions.py` wrappers (net -5 mypy no-any-return);
`endpoint_for_plane` + `get_session_bifrost` (slice-6) + the exception classes
stay. Retired the corresponding `test_sessions.py` classes; added the slice-5
adapter tests + a CLI malformed-create-abort test.

LIVE SMOKE (:8081, b128) — INV-CUT-5 / DEC-4 cleared: `--whoami` rendered real
identity + capabilities; `--characters` drove the full lifecycle end-to-end
(char-rp catalog → created char_8c00006e… → PAD read-back → deleted).

Suite 483 green; ruff clean; mypy at the 2 pre-existing baseline errors.
Patch bump 0.21.15 → 0.21.16 (the cutover MINOR is deferred to slice-7, DEC-6).

tests/test_sessions.py

@@ -6,14 +6,8 @@ import respx
 
 from ratatoskr.sessions import (
     SessionApiFailed,
-    create_character,
-    delete_character,
     endpoint_for_plane,
-    get_capabilities,
-    get_character_state,
-    get_me,
     get_session_bifrost,
-    list_character_models,
 )
 
 
@@ -38,99 +32,6 @@ class TestEndpointForPlane:
             endpoint_for_plane("persona", "10.100.10.50")
 
 
-class TestGetMe:
-    """docs/contracts/issues/2.contract.md FN get_me (slice: capabilities+me)."""
-
-    @respx.mock
-    async def test_happy_authenticated(self) -> None:
-        """happy_authenticated [happy,tracer]: 200 → parsed identity dict verbatim."""
-        respx.get("https://w.example/me").mock(
-            return_value=httpx.Response(
-                200,
-                json={
-                    "user_id": "alice",
-                    "scopes": ["conversations.read", "conversations.write"],
-                    "tier": "user",
-                    "key_id": "a1b2c3d4",
-                    "key_label": "alice phone",
-                },
-            )
-        )
-        async with httpx.AsyncClient(base_url="https://w.example") as client:
-            me = await get_me(client)
-        assert me["user_id"] == "alice"
-        assert me["tier"] == "user"
-        assert me["key_id"] == "a1b2c3d4"
-        assert me["scopes"] == ["conversations.read", "conversations.write"]
-
-    @respx.mock
-    async def test_anonymous_dev_mode(self) -> None:
-        """anonymous_dev_mode: 200 anonymous shape → dict with tier=anonymous."""
-        respx.get("https://w.example/me").mock(
-            return_value=httpx.Response(
-                200,
-                json={
-                    "user_id": "anonymous",
-                    "scopes": ["conversations.read"],
-                    "tier": "anonymous",
-                },
-            )
-        )
-        async with httpx.AsyncClient(base_url="https://w.example") as client:
-            me = await get_me(client)
-        assert me["tier"] == "anonymous"
-        assert "key_id" not in me  # optional fields omitted, not null
-
-    @respx.mock
-    async def test_401_raises_session_api_failed(self) -> None:
-        """401_raises [error]: bad/absent key → SessionApiFailed(status=401)."""
-        respx.get("https://w.example/me").mock(
-            return_value=httpx.Response(401, json={"detail": "auth_invalid"})
-        )
-        async with httpx.AsyncClient(base_url="https://w.example") as client:
-            with pytest.raises(SessionApiFailed) as exc:
-                await get_me(client)
-        assert exc.value.status == 401
-
-
-class TestGetCapabilities:
-    """docs/contracts/issues/2.contract.md FN get_capabilities (slice: capabilities+me)."""
-
-    @respx.mock
-    async def test_happy(self) -> None:
-        """happy [happy]: 200 → ephemeral_templates dict verbatim."""
-        respx.get("https://w.example/capabilities").mock(
-            return_value=httpx.Response(
-                200,
-                json={
-                    "ephemeral_templates": {
-                        "echo": {
-                            "allowed_models": ["glm5-turbo", "glm4.7"],
-                            "default_model": "glm5-turbo",
-                            "system_prompt_max_bytes": 32768,
-                        }
-                    }
-                },
-            )
-        )
-        async with httpx.AsyncClient(base_url="https://w.example") as client:
-            caps = await get_capabilities(client)
-        echo = caps["ephemeral_templates"]["echo"]
-        assert echo["default_model"] == "glm5-turbo"
-        assert echo["system_prompt_max_bytes"] == 32768
-
-    @respx.mock
-    async def test_non_200_raises(self) -> None:
-        """non_200_raises [error]: 500 → SessionApiFailed(status=500)."""
-        respx.get("https://w.example/capabilities").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:
-                await get_capabilities(client)
-        assert exc.value.status == 500
-
-
 class TestGetSessionBifrost:
     """#2 contract — get_session_bifrost (GET /admin/sessions/{id}/bifrost, #176)."""
 
@@ -191,59 +92,3 @@ 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