feat: @agor/stores package + Electrobun hardening (WIP)

- packages/stores/: theme, notifications, health stores extracted
- Electrobun hardening: durable event sequencing, file conflict detection,
  push-based updates, backpressure guards (partial, agents still running)

ui-electrobun/src/shared/pty-rpc-schema.ts

@@ -128,7 +128,12 @@ export type PtyRPCRequests = {
       error?: string;
     };
   };
-  /** Write text content to a file. */
+  /** Get file stat info (mtime, size) for conflict detection. */
+  "files.stat": {
+    params: { path: string };
+    response: { mtimeMs: number; size: number; error?: string };
+  };
+  /** Write text content to a file (atomic temp+rename). */
   "files.write": {
     params: { path: string; content: string };
     response: { ok: boolean; error?: string };