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_cli.py

@@ -1923,6 +1923,27 @@ class TestTier2Probes:
         assert "deleted: char_z" in out
         assert del_route.call_count == 1  # lifecycle cleaned up
 
+    @respx.mock
+    def test_characters_probe_create_missing_id_aborts(
+        self, capsys: pytest.CaptureFixture[str]
+    ) -> None:
+        """create ACK without character_id → clean abort (exit 20), never a hard-index
+        KeyError (open-world degrade-not-crash; slice-5 cutover foot-gun)."""
+        respx.get("https://w.example/models/available-for-characters").mock(
+            return_value=httpx.Response(200, json={"items": []})
+        )
+        # 201 but the open-world ACK omits character_id — the probe must degrade.
+        respx.post("https://w.example/characters").mock(
+            return_value=httpx.Response(201, json={"ttl_expires_at": "t"})
+        )
+        del_route = respx.delete(url__regex=r"https://w\.example/characters/.+").mock(
+            return_value=httpx.Response(204)
+        )
+        rc = main(["--characters", "--api-key", "k", "--server", "https://w.example"])
+        assert rc == 20
+        assert "no character_id" in capsys.readouterr().err
+        assert del_route.call_count == 0  # aborted before state/delete — nothing to clean
+
     @respx.mock
     def test_set_persona_probe(self, capsys: pytest.CaptureFixture[str]) -> None:
         """set_persona_probe [happy,tracer]: POST pad to /sessions/{id}/persona_state; 204."""