small bugs from the 2.8.0 docs audit: tasks_list without server_name fails; doctor ignores --lock-dir #354

Found by the 2.8.0 docs audit (README lane). Both are code bugs the README now works around or doesn't promise.

1. gateway.tasks_list with no server_name returns ok=false ("Server airbnb is not connected") instead of listing tasks across connected servers. One unconnected (lazy/offline) server aborts the whole listing. Expected: skip servers that are not connected or don't support tasks, and list the rest.
2. pmcp doctor hardcodes ~/.pmcp/gateway.lock (src/pmcp/cli.py:~2111) and ignores --lock-dir, so it reports on the wrong lock when the gateway runs with a custom lock directory.

Each needs a regression test.