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>

tests/test_guidance_ownership.py

@@ -155,6 +155,11 @@ TOPICS: tuple[Topic, ...] = (
     Topic("a retrieved rule outranks a default habit", U, ("outranks a default habit",),
           "a retrieved rule outranks a default habit"),
     Topic("log on completion and on a problem", U, ("hit or discover a problem",), "hit or discover a problem"),
+    # Milestone 381 step 4: the half of a hand-off only the model can do. The
+    # release is mechanical (SessionEnd hook); saying what happened is not.
+    Topic("hand off before the context stops existing", U,
+          ("hand off", "claims it for this session"),
+          "write down what the next session needs"),
     Topic("the project's design system binds ui", U, ("resolve_design_system",),
           "building ui: the project's design system binds", index=("resolve_design_system",)),
     Topic("name the record, never just its number", U, ("name the record",),