feat(persistence): PR6 T6.1 — ThreadMetaRepository.create workspace_id sentinel

`create()` now accepts `workspace_id: str | None | _AutoSentinel = AUTO`
on both the SQL and in-memory implementations (and the abstract base).
AUTO resolves via `resolve_workspace_id()` from the workspace contextvar
that PR4 AuthMiddleware sets; explicit None bypasses for migration paths;
explicit str overrides the contextvar.

Test infrastructure:
- conftest gains an autouse `_auto_workspace_context` fixture mirroring
  the existing user fixture. Opt-out via `@pytest.mark.no_auto_workspace`.
- A SQLAlchemy `after_create` listener on `Base.metadata` seeds the
  matching `test-workspace-autouse` + `test-user-autouse` rows whenever
  `init_engine` runs `create_all()`, so the FK from threads_meta to
  workspaces resolves. Alembic migration tests bypass create_all and are
  unaffected, keeping real FK constraints under test.
- `test_thread_meta_workspace_filter.py` covers the three AUTO / explicit
  / None paths.

3 new tests pass; 115 existing thread_meta/run/feedback/run_event/owner
tests stay green.

backend/packages/harness/deerflow/persistence/thread_meta/memory.py

@@ -13,6 +13,13 @@ from langgraph.store.base import BaseStore
 
 from deerflow.persistence.thread_meta.base import ThreadMetaStore
 from deerflow.runtime.user_context import AUTO, _AutoSentinel, resolve_user_id
+from deerflow.runtime.workspace_context import AUTO as WORKSPACE_AUTO
+from deerflow.runtime.workspace_context import (
+    _AutoSentinel as _WorkspaceAutoSentinel,
+)
+from deerflow.runtime.workspace_context import (
+    resolve_workspace_id,
+)
 from deerflow.utils.time import coerce_iso, now_iso
 
 THREADS_NS: tuple[str, ...] = ("threads",)
@@ -44,15 +51,18 @@ class MemoryThreadMetaStore(ThreadMetaStore):
         *,
         assistant_id: str | None = None,
         user_id: str | None | _AutoSentinel = AUTO,
+        workspace_id: str | None | _WorkspaceAutoSentinel = WORKSPACE_AUTO,
         display_name: str | None = None,
         metadata: dict | None = None,
     ) -> dict:
         resolved_user_id = resolve_user_id(user_id, method_name="MemoryThreadMetaStore.create")
+        resolved_workspace_id = resolve_workspace_id(workspace_id, method_name="MemoryThreadMetaStore.create")
         now = now_iso()
         record: dict[str, Any] = {
             "thread_id": thread_id,
             "assistant_id": assistant_id,
             "user_id": resolved_user_id,
+            "workspace_id": resolved_workspace_id,
             "display_name": display_name,
             "status": "idle",
             "metadata": metadata or {},