story-kit: mergemaster conflict resolution, vite proxy fix, bug 279

- Upgrade mergemaster prompt to resolve complex conflicts itself
  instead of just reporting failure. Includes instructions to check
  git history and story files for context before resolving.
- Add proxy error handler to vite config to prevent crashes on
  backend ECONNREFUSED.
- Fix bug 279: auto-assign now checks that preferred agent's stage
  matches the pipeline stage. Coders won't be assigned to QA/merge.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/vite.config.ts

@@ -16,6 +16,12 @@ export default defineConfig(() => {
         "/api": {
           target: `http://127.0.0.1:${String(backendPort)}`,
           timeout: 120000,
+          configure: (proxy) => {
+            proxy.on("error", (_err) => {
+              // Swallow proxy errors (e.g. ECONNREFUSED during backend restart)
+              // so the vite dev server doesn't crash.
+            });
+          },
         },
       },
       watch: {