Release v0.51.261 — Release IC (stage-r11) (#3616)

## Release v0.51.261 — Release IC (stage-r11)

Live Todos panel via an explicit `todo_state` SSE contract.

### Fixed
| Issue/PR | Author | Fix |
|----------|--------|-----|
| #3373 follow-up (#3454) | @v2psv | The Todos side panel now tracks `todo` tool state **live during an active run** instead of staying stale until settle / rolling back on a mid-stream reload. A dedicated `todo_state` SSE event sends a full, redacted, idempotent snapshot on todo-tool completion (no more truncated `tool_complete.preview`); the same `api.todo_state` parser feeds live + cold-load; live snapshots persist into INFLIGHT so reload/reattach restores the panel; cold-load vs INFLIGHT reconciled by timestamp (incl. the `coldTs===0` compressed-session edge); legacy reverse-scan kept as fallback for old servers. |

### Gate
- Full pytest suite: **7692 passed, 0 failed**
- ESLint: CLEAN · ruff: CLEAN · browser-smoke: CLEAN
- Codex (regression): **SAFE TO SHIP** — verified the new `todo_state` SSE handler composes with existing dispatch (no double-subscribe), INFLIGHT persistence is cleared on terminal/cancel (composes with discard_session + turn-journal), timestamp reconciliation can't let a stale local snapshot win, redaction holds, the legacy reverse-scan fallback still works with no double-render, and the `models.py` change is todo-scoped (no CLI-classification interaction).

Co-authored-by: v2psv <v2psv@users.noreply.github.com>

api/models.py

@@ -137,6 +137,14 @@ def _persisted_session_ids_snapshot() -> frozenset[str]:
     return ids
 
 
+def _session_dir_has_persisted_session_files() -> bool:
+    """Return True when the current session dir has at least one session JSON file."""
+    try:
+        return any(not p.name.startswith('_') for p in SESSION_DIR.glob('*.json'))
+    except Exception:
+        return False
+
+
 def _rebuild_session_index_background() -> None:
     try:
         _write_session_index(updates=None)
@@ -2825,6 +2833,8 @@ def all_sessions(diag=None):
             with LOCK:
                 in_memory_ids = set(SESSIONS.keys())
             persisted_ids = _persisted_session_ids_snapshot()
+            if not index and _session_dir_has_persisted_session_files():
+                raise ValueError("empty session index while session files exist")
             index = [
                 s for s in index
                 if (
@@ -2839,6 +2849,8 @@ def all_sessions(diag=None):
                     )
                 )
             ]
+            if not index and _session_dir_has_persisted_session_files():
+                raise ValueError("session index has no live rows while session files exist")
             backfilled = []
             for i, s in enumerate(index):
                 if 'last_message_at' not in s: