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.

tests/95-stub-roundtrip.bats

@@ -564,7 +564,7 @@ STUB
   # deliberately do NOT auto-clean: the same code path triggers on a
   # concurrent in-flight host attempt for the same name, and racing
   # rm -rf calls would wipe each other's fresh dirs.
-  # Recovery is a manual `rm -rf` per the error message.
+  # Recovery is `party clean` per the error message.
   party_dir="$PARTY_SOCKET_DIR/party-$USER:resurrect.d"
   mkdir -p "$party_dir"
   : > "$party_dir/sock"
@@ -572,7 +572,7 @@ STUB
   run "$PARTY_BIN" host resurrect
   [ "$status" -ne 0 ]
   [[ "$output" == *"already exists"* ]]
-  [[ "$output" == *"rm -rf $party_dir"* ]]
+  [[ "$output" == *"party clean"* ]]
 
   # The leftover dir is untouched — proves we did not race-clean.
   [ -d "$party_dir" ]