feat: Add robust fallback mechanisms for spec creation chat

Add multiple escape hatches to prevent users from getting stuck during
spec creation when the WebSocket completion signal fails.

Changes:
- Add "Exit to Project" button always visible in chat header
- Add /exit command detection to immediately exit to project
- Add backend GET /api/spec/status/{project} endpoint to poll status file
- Add getSpecStatus() API function in frontend
- Add status file polling (every 3s) in useSpecChat hook
- Update create-spec.md with status file write instructions

How it works:
1. Happy path: Claude writes .spec_status.json as final step, UI polls
   and detects completion, shows "Continue to Project" button
2. Escape hatch: User can always click "Exit to Project" or type /exit
   to instantly select the project and close modal, then manually start
   the agent from the main UI

This ensures users always have a way forward even if the WebSocket
completion detection fails due to tool call tracking issues.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/lib/api.ts

@@ -145,6 +145,22 @@ export async function resumeAgent(projectName: string): Promise<AgentActionRespo
   })
 }
 
+// ============================================================================
+// Spec Creation API
+// ============================================================================
+
+export interface SpecFileStatus {
+  exists: boolean
+  status: 'complete' | 'in_progress' | 'not_started' | 'error' | 'unknown'
+  feature_count: number | null
+  timestamp: string | null
+  files_written: string[]
+}
+
+export async function getSpecStatus(projectName: string): Promise<SpecFileStatus> {
+  return fetchJSON(`/spec/status/${encodeURIComponent(projectName)}`)
+}
+
 // ============================================================================
 // Setup API
 // ============================================================================