feat: party clean removes the caller's own crash leftovers

Turns the manual rm -rf recovery from host's EEXIST message into a
verb, behind the same gates cmd_close uses: own-basename glob, no
symlinks, ownership check, and a dead socket. Live parties are skipped
with a pointer to party close.

ROADMAP.md

@@ -16,9 +16,6 @@ Direction, not promises. Two rules govern everything below (README
   per-party dir (group-readable so every attendee can grab a copy);
   `party log --stop` ends it. "What did we do last night?" for
   collectives, and the audit trail human+AI co-work needs.
-- **`party clean`:** remove the caller's own crash leftovers (dirs
-  they own whose socket is dead), turning the manual `rm -rf` recovery
-  in host's error message into a verb.
 
 ## Later