fix: handle stale locks and missing ui_messages.json on startup

[roomote]

Summary

This PR fixes the gray screen issue that occurs when ui_messages.json is missing but lock files remain in the task directory.

Problem

When VS Code crashes or the Remote Extension Host is killed mid-task:

• Lock files remain in the task directory
• ui_messages.json may be missing
• On reload, Roo Code tries to acquire locks but fails repeatedly with "Lock file is already being held"
• The UI becomes unresponsive (gray screen)

Solution

1. Startup Scan: Added stale lock detection on extension activation that scans task directories for orphaned locks
2. Automatic Recovery:
   • Removes lock files older than 10 minutes when ui_messages.json is missing
   • Creates an empty ui_messages.json to restore functionality
3. Preventive Guard: Updated safeWriteJson to create the target file before lock acquisition

Changes

• Added staleLockRecovery.ts utility module with detection and recovery logic
• Integrated stale lock recovery into extension activation
• Modified safeWriteJson to handle missing files gracefully
• Added comprehensive tests for all new functionality

Testing

• ✅ All existing tests pass
• ✅ Added new tests for stale lock detection and recovery
• ✅ Added tests for safeWriteJson file creation behavior
• ✅ Linting and type checking pass

Fixes #6022

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Important

Fixes gray screen issue by detecting and recovering from stale lock files and missing ui_messages.json on startup.

• Behavior:
  • On startup, detects and recovers from stale lock files in task directories if ui_messages.json is missing.
  • Removes lock files older than 10 minutes and creates an empty ui_messages.json to restore functionality.
  • Integrated recovery process into extension activation in extension.ts.
• Utilities:
  • Added staleLockRecovery.ts for stale lock detection and recovery logic.
  • Modified safeWriteJson to create target file before lock acquisition.
• Testing:
  • Added tests for stale lock detection and recovery in staleLockRecovery.test.ts.
  • Added tests for safeWriteJson behavior in safeWriteJson.staleLock.test.ts.

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