feat(#20): worldtree-sdk adapter foundation — ratatoskr.wt (slice-1)

Slice-1 of the SDK cutover (docs/contracts/worldtree_sdk_cutover.contract.md):
the adapter chokepoint onto worldtree-sdk 1.0.0, unit-tested but not yet wired
to any surface (that is slice-2).

- build_client(base_url, *, api_key, admin_key=None, transport) constructs the
  single WorldtreeClient over a ratatoskr-owned injected httpx.AsyncClient.
  INV-CUT-1: the SDK is given the transport (_owns_client=False) and never closes
  it — proven by a test asserting aclose() leaves ratatoskr's transport open.
- translate_error implements the § Error map DEFAULT: SDK ApiError → the adapter's
  SessionApiFailed (carrying the SDK's parsed status/error_code/body); every
  discriminated WorldtreeError subclass passes through by identity. Route-specific
  rows land at their call-sites in later slices (the route is the discriminator).
- SessionApiFailed gains error_code vs the retiring sessions.py copy (extends it
  per the contract error-map row); the two coexist transiently and reconcile in
  slice-2 (DEC-4 incremental cutover — nothing wires the adapter this slice, so
  they never meet at runtime).

Deletes no hand-rolled path, so DEC-4's live-smoke bar does not apply yet.
Suite 541 green (534 + 7 new); mypy + ruff clean. Patch (internal foundation;
the cutover's minor bump is DEC-6 at slice-7 ship).

tests/test_wt.py

@@ -0,0 +1,85 @@
+"""Unit tests for the worldtree-sdk adapter (`ratatoskr.wt`) — slice-1 foundation.
+
+Covers the two foundation surfaces (issue #20 cutover contract, slice 1):
+  * `build_client` — construction wiring + injected-transport ownership (INV-CUT-1:
+    the SDK must never close ratatoskr's transport).
+  * `translate_error` — the § Error map DEFAULT (`ApiError` → `SessionApiFailed`)
+    plus discriminated-`WorldtreeError` passthrough (INV-CUT-2).
+
+No ratatoskr surface (CLI / web / TUI) is exercised here — that wiring lands in
+slice 2. These tests hit no network (WorldtreeClient does no I/O at construction).
+"""
+
+from __future__ import annotations
+
+import httpx
+from worldtree_sdk import AgentNotAvailable, ApiError, WorldtreeClient
+
+from ratatoskr.wt import SessionApiFailed, build_client, translate_error
+
+
+class TestBuildClient:
+    async def test_constructs_worldtree_client(self) -> None:
+        transport = httpx.AsyncClient()
+        try:
+            client = build_client(
+                "https://wt.example:8081", api_key="ck-test", transport=transport
+            )
+            assert isinstance(client, WorldtreeClient)
+            assert client.base_url == "https://wt.example:8081"
+        finally:
+            await transport.aclose()
+
+    async def test_injected_transport_is_ratatoskr_owned(self) -> None:
+        # INV-CUT-1 [hard]: aclose() on the SDK client must NOT close ratatoskr's
+        # transport — ratatoskr owns the lifecycle exactly as it does today.
+        transport = httpx.AsyncClient()
+        client = build_client("https://wt.example", api_key="ck", transport=transport)
+        await client.aclose()
+        assert client.closed is True
+        assert transport.is_closed is False
+        await transport.aclose()
+
+    async def test_admin_key_optional(self) -> None:
+        transport = httpx.AsyncClient()
+        try:
+            # Absent admin_key → admin_auth=None; still constructs.
+            without_admin = build_client(
+                "https://wt.example", api_key="ck", transport=transport
+            )
+            assert isinstance(without_admin, WorldtreeClient)
+            # Present admin_key → constructs (admin surface available in later slices).
+            with_admin = build_client(
+                "https://wt.example", api_key="ck", admin_key="ak", transport=transport
+            )
+            assert isinstance(with_admin, WorldtreeClient)
+        finally:
+            await transport.aclose()
+
+
+class TestTranslateError:
+    def test_apierror_maps_to_session_api_failed_default(self) -> None:
+        exc = ApiError("some_code", "boom", status=500, body="raw-body")
+        mapped = translate_error(exc)
+        assert isinstance(mapped, SessionApiFailed)
+        assert mapped.status == 500
+        assert mapped.error_code == "some_code"
+        assert mapped.body == "raw-body"
+
+    def test_apierror_with_no_body_maps_cleanly(self) -> None:
+        exc = ApiError("nope", "no body", status=404)
+        mapped = translate_error(exc)
+        assert isinstance(mapped, SessionApiFailed)
+        assert mapped.status == 404
+        assert mapped.error_code == "nope"
+        assert mapped.body is None
+
+    def test_discriminated_subclass_passes_through_unchanged(self) -> None:
+        # Discriminated WorldtreeError subclasses are already the right semantic
+        # type — the adapter passes them through by identity (no re-wrap).
+        exc = AgentNotAvailable("agent_not_available", "gone", status=409)
+        assert translate_error(exc) is exc
+
+    def test_non_worldtree_error_passes_through_unchanged(self) -> None:
+        exc = ValueError("unrelated")
+        assert translate_error(exc) is exc