Summary

Fixes #13511

After an abnormal gateway exit (SIGKILL, crash, OOM, terminal disconnect), the gateway.pid file remains on disk. On next gateway start, write_pid_file() uses O_CREAT | O_EXCL which raises FileExistsError — causing the gateway to exit with "PID file race lost to another gateway instance" even though the old process is dead.

Root Cause

write_pid_file() in gateway/status.py does not check whether the process recorded in a pre-existing PID file is still alive before attempting the atomic O_EXCL create. The FileExistsError is unconditionally re-raised, blocking restart.

Fix

Before the os.open(path, O_CREAT | O_EXCL | ...) call, add a stale PID cleanup step:

1. If the PID file exists, read the recorded PID
2. Probe the process with os.kill(pid, 0)
3. If ProcessLookupError (process dead), remove the stale file
4. Then proceed with the normal O_EXCL creation

This preserves the atomic race behavior for truly concurrent starts while fixing the stale-file blocking case.

Testing

Added 2 new tests in tests/gateway/test_status.py:

• test_write_pid_file_cleans_stale_pid_from_dead_process — stale PID file is cleaned, write succeeds
• test_write_pid_file_preserves_pid_file_for_live_process — live process PID file preserved, FileExistsError raised

All 13 TestGatewayPidState tests pass.