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>

plugin/skills/using-scribe/SKILL.md

@@ -175,6 +175,16 @@ Two constraints on *how* that's achieved:
    **complete** a task and when you **hit or discover a problem**, so a change
    of direction is on the record and not only the successes.
 
+   **Hand off before this session's context stops existing.** A compaction, a
+   `/clear`, the operator wrapping up for the day — each is the last moment the
+   reasoning behind the work lives anywhere but here. Log on the task you were
+   holding where it stands, what you tried and ruled out, and the next move:
+   write down what the next session needs, because it arrives with Scribe's
+   record and nothing else. Moving a task to `in_progress` or logging on it also
+   claims it for this session — that claim is what hands the work back to you
+   after a compaction, and it ends on its own when you stop, so there is nothing
+   to release by hand.
+
 6. **Fixes are issues, not work-logs.** When you fix a problem — even one solved
    in passing — record it as its own issue (`create_task(kind="issue")`) with
    symptom → root cause → fix, optionally linked to the task it arose from