Fix _save_state race condition losing last_reflection timestamp

Daily and weekly timers fired simultaneously on Sundays, causing
decay() and reflect() to race on _state.json. Now merges top-level
keys before writing and uses atomic tempfile+rename to prevent
partial reads from triggering silent JSONDecodeError fallbacks.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills/cognitive-memory/client.py

@@ -622,9 +622,30 @@ class CognitiveMemoryClient:
         return {"version": 1, "updated": "", "entries": {}}
 
     def _save_state(self, state: Dict):
-        """Write _state.json."""
+        """Write _state.json atomically, merging top-level keys to prevent race conditions."""
+        import tempfile
+
+        # Merge with existing state to preserve keys written by concurrent processes
+        if self.state_path.exists():
+            try:
+                existing = json.loads(self.state_path.read_text())
+                existing.update(state)
+                state = existing
+            except (json.JSONDecodeError, OSError):
+                pass
         state["updated"] = datetime.now(timezone.utc).isoformat()
-        self.state_path.write_text(json.dumps(state, indent=2, default=str))
+        # Atomic write: write to temp file then rename
+        fd, tmp_path = tempfile.mkstemp(dir=self.memory_dir, suffix=".tmp")
+        try:
+            with os.fdopen(fd, "w") as f:
+                json.dump(state, f, indent=2, default=str)
+            os.replace(tmp_path, self.state_path)
+        except Exception:
+            try:
+                os.unlink(tmp_path)
+            except OSError:
+                pass
+            raise
 
     # -------------------------------------------------------------------------
     # File I/O