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/bun/handlers/files-handlers.ts

@@ -76,6 +76,21 @@ export function createFilesHandlers() {
       }
     },
 
+    // Feature 2: Get file stat (mtime) for conflict detection
+    "files.stat": async ({ path: filePath }: { path: string }) => {
+      const guard = guardPath(filePath);
+      if (!guard.valid) {
+        return { mtimeMs: 0, size: 0, error: guard.error };
+      }
+      try {
+        const stat = fs.statSync(guard.resolved);
+        return { mtimeMs: stat.mtimeMs, size: stat.size };
+      } catch (err) {
+        const error = err instanceof Error ? err.message : String(err);
+        return { mtimeMs: 0, size: 0, error };
+      }
+    },
+
     "files.write": async ({ path: filePath, content }: { path: string; content: string }) => {
       const guard = guardPath(filePath);
       if (!guard.valid) {
@@ -83,7 +98,10 @@ export function createFilesHandlers() {
         return { ok: false, error: guard.error };
       }
       try {
-        fs.writeFileSync(guard.resolved, content, "utf8");
+        // Feature 2: Atomic write via temp file + rename
+        const tmpPath = guard.resolved + ".agor-tmp";
+        fs.writeFileSync(tmpPath, content, "utf8");
+        fs.renameSync(tmpPath, guard.resolved);
         return { ok: true };
       } catch (err) {
         const error = err instanceof Error ? err.message : String(err);