Stale LOCK file blocks all commands after process kill

If a tgcli process is killed (timeout, SIGKILL, crash), the LOCK file remains and blocks all subsequent commands.

**Repro:**
```bash
tgcli send text --to @someone --message 'test' &
kill -9 $!
tgcli doctor --json
# → {"ok":false,"error":"Store is locked by another process ..."}
```

**Workaround:** `rm ~/.local/share/tgcli/LOCK`

**Expected:** Auto-detect stale locks (check if PID is alive).