feat(tasks): a task can say a session is working it — the claim (milestone 381 step 2)

status is durable and nothing clears it, so in_progress cannot also mean
"someone is on this now". The claim is that second meaning, stored as who
and when so it dies on read rather than needing to be cleared.

- notes.claimed_by / claimed_at / claim_touched_at / claim_session (0110).
- The server stamps it on the write that is the work: reaching in_progress
  (update or create, via apply_status_transition) and a work log on an open
  task. done/cancelled/todo release it. Live while touched within
  CLAIM_LEASE (2h); dead on read past it, with no sweep.
- The plugin's PostToolUse hook on update_task/add_task_log binds the
  harness's session_id (GET /api/plugin/claim-session). It binds only to a
  live claim the caller holds; it cannot create one.
- to_dict carries `claim`. Plugin version minted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/services/notes.py

@@ -308,23 +308,32 @@ def build_note(
     # (#3683) — otherwise `create_task(status="in_progress")` writes a row no
     # update could produce: started, with no `started_at`.
     if status is not None:
-        apply_status_transition(note)
+        apply_status_transition(note, user_id)
     return note
 
 
-def apply_status_transition(note: Note) -> None:
+def apply_status_transition(note: Note, user_id: int | None = None) -> None:
     """Stamp what reaching `note.status` implies — the ONE statement of it.
 
     Called by the update path whenever `status` is written and by `build_note`
     whenever a create names one, so a task created at a status is
     indistinguishable from one that reached it by update. Two copies of "what
     a status implies" are where the two drift, and #3683 was that drift.
+
+    `user_id` is whoever is making the change: reaching `in_progress` is the
+    moment a session takes the work on, so it stamps that user's claim, and a
+    status that ends or un-starts the work releases it (milestone 381).
     """
+    from scribe.services.task_claims import release_claim, stamp_claim
+
     _now = datetime.now(timezone.utc)
     if note.status == TaskStatus.in_progress.value:
         if note.started_at is None:
             note.started_at = _now
+        if user_id is not None:
+            stamp_claim(note, user_id, _now)
     elif note.status in (TaskStatus.done.value, TaskStatus.cancelled.value):
+        release_claim(note)
         note.completed_at = _now
         if note.recurrence_rule:
             from scribe.services.recurrence import calculate_next_due
@@ -334,6 +343,7 @@ def apply_status_transition(note: Note) -> None:
                 next_due.year, next_due.month, next_due.day, tzinfo=timezone.utc
             )
     elif note.status == TaskStatus.todo.value:
+        release_claim(note)
         note.started_at = None
         note.completed_at = None
         note.recurrence_next_spawn_at = None
@@ -687,7 +697,7 @@ async def update_note(
             if recompose is not None:
                 note.data = recompose(note)
         if "status" in fields:
-            apply_status_transition(note)
+            apply_status_transition(note, user_id)
         note.updated_at = datetime.now(timezone.utc)
         await session.commit()
         await session.refresh(note)