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/sql.py

@@ -11,6 +11,13 @@ from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 from deerflow.persistence.thread_meta.base import ThreadMetaStore
 from deerflow.persistence.thread_meta.model import ThreadMetaRow
 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,
+)
 
 
 class ThreadMetaRepository(ThreadMetaStore):
@@ -33,17 +40,21 @@ class ThreadMetaRepository(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:
-        # Auto-resolve user_id from contextvar when AUTO; explicit None
-        # creates an orphan row (used by migration scripts).
+        # Auto-resolve both user_id and workspace_id from contextvars when
+        # AUTO; explicit None creates an orphan row (used by migration
+        # scripts that intentionally bypass scope).
         resolved_user_id = resolve_user_id(user_id, method_name="ThreadMetaRepository.create")
+        resolved_workspace_id = resolve_workspace_id(workspace_id, method_name="ThreadMetaRepository.create")
         now = datetime.now(UTC)
         row = ThreadMetaRow(
             thread_id=thread_id,
             assistant_id=assistant_id,
             user_id=resolved_user_id,
+            workspace_id=resolved_workspace_id,
             display_name=display_name,
             metadata_json=metadata or {},
             created_at=now,