feat: party knock asks an invite-only party's host for an invite

party list already shows invite-only parties; knock closes the loop.
The knocker can't run any tmux command against the server, so the
channel is write(1), the same one invite uses toward guests, except
here delivery failure is surfaced: the ping is the whole feature.

ROADMAP.md

@@ -12,10 +12,6 @@ Direction, not promises. Two rules govern everything below (README
 
 ## Next: small, high value
 
-- **`party knock <name>`:** an uninvited group member pings the host
-  via `write(1)`: "veg wants to join fiesta". `party list` already
-  shows invite-only parties; knock completes that loop. The feature
-  most likely to cause spontaneous pairing.
 - **`party log`:** per-party transcript via `tmux pipe-pane` into the
   per-party dir (group-readable so every attendee can grab a copy);
   `party log --stop` ends it. "What did we do last night?" for