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_task_claims.py

@@ -256,3 +256,30 @@ async def test_session_start_after_a_compaction_carries_the_claimed_task(users):
         owner, source="compact", session_id="sess-compact"))["context"]
     assert "claimed then compacted" in ctx
     assert "the migration is written" in ctx
+
+
+# --- step 4: the hand-off ----------------------------------------------------
+
+def test_session_end_releases_claims_but_not_on_clear():
+    """The mechanical half of the hand-off. A /clear keeps the claim, because
+    SessionStart(source=clear) pushes the claimed work straight back."""
+    hooks = json.loads((ROOT / "plugin/hooks/hooks.json").read_text())["hooks"]
+    commands = [h["command"] for b in hooks.get("SessionEnd", []) for h in b["hooks"]]
+    assert any("scribe_session_end.sh" in c for c in commands)
+    text = (ROOT / "plugin/hooks/scribe_session_end.sh").read_text()
+    assert '[ "$reason" = "clear" ] && exit 0' in text
+    assert "/api/plugin/release-session" in text
+
+
+@pytest.mark.integration
+async def test_ending_a_session_releases_only_that_sessions_claims(users):
+    owner, _ = users
+    kept = await notes_svc.create_note(owner, title="other session's", status="in_progress")
+    gone = await notes_svc.create_note(owner, title="ending session's", status="in_progress")
+    await tc.bind_session(owner, kept.id, "sess-stays")
+    await tc.bind_session(owner, gone.id, "sess-ends")
+
+    assert await tc.release_session(owner, "sess-ends") >= 1
+    assert (await notes_svc.get_note(owner, gone.id)).claimed_at is None
+    assert (await notes_svc.get_note(owner, kept.id)).claim_session == "sess-stays"
+    assert await tc.release_session(owner, "") == 0