Add lock mechanism to prevent concurrent access to worktrees:
- Add LOCKS_DIR constant (~/.kugetsu/locks)
- Add acquire_lock() - acquires lock file with PID, session_id, timestamp
- Add release_lock() - releases lock if held by current process
- Add release_all_locks() - releases all locks for a session
- Add check_lock() - check if issue is locked
- Modify cmd_start to acquire lock before creating worktree
- Modify cmd_destroy to release lock when destroying session
- Add trap for cleanup on EXIT/INT/TERM
Lock files named after issue ref. Stale lock detection: if PID
no longer exists, lock is considered stale.
Fixes#71
