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/mainview/FileBrowser.svelte

@@ -32,6 +32,9 @@
   let editorContent = $state('');
   // Fix #6: Request token to discard stale file load responses
   let fileRequestToken = 0;
+  // Feature 2: Track mtime at read time for conflict detection
+  let readMtimeMs = $state(0);
+  let showConflictDialog = $state(false);
 
   // Extension-based type detection
   const CODE_EXTS = new Set([