Follow-up on top of opriz's atomic PID file fix. The prior change caught
the race AFTER runner.start(), so the loser still opened Telegram polling
and Discord gateway sockets before detecting the conflict and exiting.
Hoist the PID-claim block to BEFORE runner.start(). Now the loser of the
O_CREAT|O_EXCL race returns from start_gateway() without ever bringing up
any platform adapter — no Telegram conflict, no Discord duplicate session.
Also add regression tests:
- test_write_pid_file_is_atomic_against_concurrent_writers: second
write_pid_file() raises FileExistsError rather than clobbering.
- Two existing replace-path tests updated to stateful mocks since the
real post-kill state (get_running_pid None after remove_pid_file)
is now exercised by the hoisted re-check.
ce9c91c8f7