fix: lowercase container name filter for case-sensitive docker ps

- get_container_id() and get_container_name() now lowercase the
  instance name before passing to docker ps --filter, because
  Docker container names are lowercase internally and the filter
  is case-sensitive. This caused container_id to never be captured
  when instance.name contained uppercase chars (e.g. 'Headquarter'),
  breaking terminal WebSocket connections.

- Also guard proc.stdout being None in start_cloudflared_tunnel().

- Add unit tests for get_container_id and get_container_name.

Quality gates: pytest (14 passed), python clean

.gitignore

@@ -49,3 +49,5 @@ apps/web/dist/
 .DS_Store
 Thumbs.db
 /.stoneforge/.worktrees/
+# Local Pi runtime state
+.atl/