feat(tasks): the hand-off — SessionEnd releases a session's claims; the practice is written down (milestone 381 step 4)

Release is the mechanical half: scribe_session_end.sh sends the ending
session's id to /api/plugin/release-session, which releases the claims it
held. A tidy-up, not the guarantee (no SessionEnd on a crash; the lease
covers that), and skipped on /clear so SessionStart(clear) can still
hand the claimed work back.

Saying what happened is the half only the model can do. It is stated as a
practice where it is read: the using-scribe skill owns it ("Hand off before
this session's context stops existing", pinned in test_guidance_ownership),
the static context points at it for the wrap-up moment, and add_task_log's
docstring says a log claims the task. _INSTRUCTIONS is untouched.

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

src/scribe/mcp/tools/tasks.py

@@ -483,6 +483,11 @@ async def add_task_log(task_id: int, content: str) -> dict:
     cannot get from the body or the diff is what you tried, what you ruled
     out, and where it actually stands.
 
+    A log on an open task also marks it as being worked by this session — its
+    `claim` (milestone 381). That is what brings the task and its newest
+    entries back to you after a compaction; it lapses by itself once the
+    session stops writing, and a status of done, cancelled or todo clears it.
+
     The response shows the task's `systems` — or, if the task is an untagged
     project record, the `systems_hint` question: logging work IS working in
     some area, so answer it (update_task with system_ids, or create_system