Implement debug logfile foundation for routing internal diagnostics
to a per-session log file instead of polluting the terminal.
Core changes:
- Add DebugLogger interface and createDebugLogger() in debugLogger.ts
- Add Storage.getGlobalDebugDir() and getDebugLogPath() for log paths
- Add Config.getDebugLogger() method with session-scoped logger
- Track write failures via isDebugLoggingDegraded()
CLI changes:
- Add DebugModeNotification component for interactive startup notice
- Add non-interactive debug notice to stderr in gemini.tsx
Log files are written to ~/.qwen/debug/<sessionId>.txt with format:
2026-01-24T10:30:00.000Z [LEVEL] [TAG] message
Debug logging is always-on; debug mode only controls the startup notice.
