feat(tasks): sequence is the bar — strict sibling ordering at the claim chokepoint (#452)

* feat(tasks): enforce sibling sequence order at the claim chokepoint

A task with a parent and effective sequence N (COALESCE(sequence, 0))
can no longer be claimed while any sibling with a strictly lower
effective sequence is non-terminal — assignee-blind, independent of and
stricter than dependency_ids, enforced in _validate_claim_preconditions
so both claim paths (gateway verbs and the dispatcher's raw REST claim)
cross it. Ties run parallel; cancelled siblings never block; sequence 0
and parentless tasks are unaffected. Live failure this guards: a PM
delegated revision subtasks sequenced 0..3 with no dependency edges and
seq 2 started alongside seq 0 — sequence was advisory-only.

set_sequence's contract updated accordingly. New e2e smoke case drives
the refusal and the post-completion claim through the real gateway.

* chore(scripts): skip .uv-cache and .claude in the prose scanner

Repo-local tool dirs (private uv cache, agent worktrees) carry vendored
and generated markdown that tripped make reflow-check.

* fix(tasks): wave-derived delegation sequences + claim-gate hardening

Three fixes from the adversarial review of the sequence claim gate:

Delegation no longer stamps a raw per-sibling ordinal (deterministic
merge-order bookkeeping) as sequence — under the strict gate that
serialized ALL delegated work, including fully independent cross-dev and
cross-cell siblings. Sequences are now wave-derived post-wiring
(stamp_wave_sequence: 1 + max same-parent dependency sequence, 0 when
independent), so independent siblings tie and run parallel while
colliding/ordered work ascends. The cross-cell UX wiring restamps
instead of writing relative ux+1 values (a relative write could invert
a collision-derived stamp), and the dispatch merge/lane barriers gain a
created_at tiebreak for wave-tied siblings so shared-branch merge order
stays deterministic. PM-authored sequences are never rewritten.

The guard now also fires on reclaims from needs_revision (a lower-
sequence sibling delegated after the first claim was invisible), and
tasks.parent_task_id gains an index (migration 069) — the guard's
sibling probe ran as a Seq Scan on the hottest verb.

---------

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

tests/unit/services/test_task.py

@@ -1352,9 +1352,12 @@ async def test_unclaimed_parent_acs_counts_live_children_not_just_completed() ->
 
 
 def _svc_with_sibling_status_seq(rows: list[tuple]) -> TaskService:
-    """TaskService whose execute() yields (status, sequence) sibling rows."""
+    """TaskService whose execute() yields (status, sequence[, created_at])
+    sibling rows; 2-tuples are padded with created_at=None (only compared on
+    a sequence tie)."""
+    row_width = 3  # (status, sequence, created_at)
     res = MagicMock()
-    res.all.return_value = rows
+    res.all.return_value = [r if len(r) == row_width else (*r, None) for r in rows]
     return TaskService(MagicMock(execute=AsyncMock(return_value=res)))
 
 
@@ -1386,6 +1389,25 @@ async def test_earlier_incomplete_code_sibling_false_when_earlier_terminal() ->
     assert await svc.has_earlier_incomplete_code_sibling(task) is False
 
 
+@pytest.mark.asyncio
+async def test_earlier_incomplete_code_sibling_tie_breaks_by_created_at() -> None:
+    # Wave ties (equal sequence) order by created_at: the earlier-created
+    # tied sibling holds the later one; a later-created one does not.
+    task = _build_task(
+        task_type=TaskType.CODE.value,
+        parent_task_id=uuid4(),
+        assigned_to=uuid4(),
+        sequence=1,
+        created_at=datetime(2026, 7, 10, 12, 0, tzinfo=UTC),
+    )
+    earlier = datetime(2026, 7, 10, 11, 0, tzinfo=UTC)
+    later = datetime(2026, 7, 10, 13, 0, tzinfo=UTC)
+    svc = _svc_with_sibling_status_seq([(TaskStatus.IN_PROGRESS, 1, earlier)])
+    assert await svc.has_earlier_incomplete_code_sibling(task) is True
+    svc = _svc_with_sibling_status_seq([(TaskStatus.IN_PROGRESS, 1, later)])
+    assert await svc.has_earlier_incomplete_code_sibling(task) is False
+
+
 @pytest.mark.asyncio
 async def test_earlier_incomplete_code_sibling_false_for_higher_seq_only() -> None:
     # A LATER sibling (seq 3) does not hold an earlier leaf (seq 2).