fix(kanban): detect cycles in decompose_triage_task sibling-link pre-validation

decompose_triage_task inlines SQL INSERTs for atomicity and intentionally
bypasses link_tasks() — which calls _would_cycle() per edge.  If the LLM
emits a cyclic parent graph (e.g. A.parents=[1], B.parents=[0]) the DB
write succeeds but every involved child deadlocks in 'todo' forever:
recompute_ready() requires all parents to be done, which is impossible
when A waits for B and B waits for A.

Add a Kahn topological sort over the sibling parent indices in the
pre-validation block, before any DB writes.  Mirrors the cycle-safety
guarantee that link_tasks() provides for manually linked tasks.

hermes_cli/kanban_db.py

@@ -2849,6 +2849,29 @@ def decompose_triage_task(
             if p == idx:
                 raise ValueError(f"child[{idx}] cannot list itself as a parent")
 
+    # Detect cycles in the sibling parent graph (Kahn's topological sort).
+    # link_tasks() calls _would_cycle() for every new edge; here we check
+    # the entire sibling graph before touching the DB.  A cycle silently
+    # deadlocks every involved child in 'todo' because recompute_ready()
+    # can never promote them.
+    _in_deg = [0] * len(children)
+    _adj: list[list[int]] = [[] for _ in range(len(children))]
+    for _i, _c in enumerate(children):
+        for _p in (_c.get("parents") or []):
+            _adj[_p].append(_i)
+            _in_deg[_i] += 1
+    _queue = [_i for _i in range(len(children)) if _in_deg[_i] == 0]
+    _seen = 0
+    while _queue:
+        _node = _queue.pop()
+        _seen += 1
+        for _nb in _adj[_node]:
+            _in_deg[_nb] -= 1
+            if _in_deg[_nb] == 0:
+                _queue.append(_nb)
+    if _seen != len(children):
+        raise ValueError("cyclic dependency detected in decomposed children list")
+
     # We do the full decomposition in a SINGLE write_txn so it's
     # atomic: either every child is created AND the root flips to
     # ``todo``, or nothing changes. We deliberately do NOT call any

tests/hermes_cli/test_kanban_decompose_db.py

@@ -132,6 +132,22 @@ def test_decompose_rejects_out_of_range_parent(kanban_home):
             )
 
 
+def test_decompose_rejects_cyclic_parents(kanban_home):
+    with kb.connect() as conn:
+        tid = _create_triage(conn)
+        with pytest.raises(ValueError, match="cyclic dependency"):
+            kb.decompose_triage_task(
+                conn,
+                tid,
+                root_assignee="orch",
+                children=[
+                    {"title": "A", "parents": [1]},
+                    {"title": "B", "parents": [0]},
+                ],
+                author="me",
+            )
+
+
 def test_decompose_records_audit_comment_and_event(kanban_home):
     with kb.connect() as conn:
         tid = _create_triage(conn)