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/models/note.py

@@ -76,6 +76,16 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
     recurrence_next_spawn_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True
     )
+    # The claim — which session is working this task NOW (milestone 381).
+    # Orthogonal to `status`: status is the work's state and durable, the claim
+    # is a session's attention and dies on read once `claim_touched_at` is past
+    # the lease. Semantics in services/task_claims.py.
+    claimed_by: Mapped[int | None] = mapped_column(
+        Integer, ForeignKey("users.id", ondelete="SET NULL"), nullable=True
+    )
+    claimed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    claim_touched_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    claim_session: Mapped[str | None] = mapped_column(Text, nullable=True)
     # WHAT KIND of record this is, on the note/entity axis. Task-ness is tracked
     # by `status`, not here (person/place/list entity types removed 2026-07):
     #   note    (default) — authored prose, findable by what it is ABOUT
@@ -184,6 +194,7 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
             "completed_at": iso(self.completed_at),
             "recurrence_rule": self.recurrence_rule,
             "recurrence_next_spawn_at": iso(self.recurrence_next_spawn_at),
+            "claim": _claim_state(self),
             "is_task": self.is_task,
             "note_type": self.note_type or "note",
             "task_kind": self.task_kind,
@@ -198,3 +209,9 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
             "created_at": iso(self.created_at),
             "updated_at": iso(self.updated_at),
         }
+
+
+def _claim_state(note: "Note") -> dict | None:
+    from scribe.services.task_claims import claim_state
+
+    return claim_state(note)