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)

packages/stores/index.ts

@@ -0,0 +1,5 @@
+// @agor/stores — shared store modules for Tauri and Electrobun frontends
+
+export * from './theme.svelte';
+export * from './notifications.svelte';
+export * from './health.svelte';