`get()` accepts `workspace_id: str | None | _AutoSentinel = AUTO` and
moves the workspace check into the SQL WHERE so cross-workspace lookups
short-circuit without loading the row. `MemoryThreadMetaStore._get_owned_record`
gets the same filter for parity.
The user_id check stays as a post-load comparison (preserves the existing
shared-row semantics where row.user_id IS NULL means "everyone in this
workspace"). Cross-workspace always returns None, never the row.
3 new tests cover the three states: in-workspace get returns the row,
out-of-workspace get returns None even when user_id matches, explicit
workspace_id=None bypasses (migration / CLI). Existing 22 thread_meta
tests stay green.
1445043649
296a4f1950