feat(agent): add git diff viewer and improve permission handling

- Add git diff viewer screen accessible via agent dropdown menu
- Implement git_diff_request/response WebSocket messages
- Add dropdown menu to agent screen header with "View Changes" option
- Clear pending permissions when starting new agent turn to prevent conflicts
- Treat agent refusals as completed rather than failed state
- Add test coverage for permission handling edge cases
- Configure vitest to run in single-threaded mode for stability
- Remove redundant wrapper in agent stream view

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

packages/server/vitest.config.ts

@@ -6,5 +6,11 @@ export default defineConfig({
     hookTimeout: 30000,
     globals: true,
     environment: "node",
+    pool: "threads",
+    poolOptions: {
+      threads: {
+        singleThread: true,
+      },
+    },
   },
 });