feat: party log records a group-readable transcript

tmux pipe-pane on every pane of the party session, appending into
<party dir>/log (0640, party group) so any attendee can grab a copy.
Start and stop are announced to everyone attached; close rescues a
non-empty transcript to the host's home before removing the dir.
Panes opened later are picked up by re-running party log (a
pane_pipe check keeps already-piped panes single-piped, since tmux's
pipe-pane -o toggles an existing pipe closed rather than skipping it).
Raw output, escapes included. Close polls the transcript for size
stability before rescuing it, since a cross-filesystem mv is
copy+unlink and could otherwise drop bytes still draining from the
pipe-pane writers.

ROADMAP.md

@@ -10,13 +10,6 @@ Direction, not promises. Two rules govern everything below (README
   encouraging people to work together in a terminal, not by adding
   machinery.
 
-## Next: small, high value
-
-- **`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
-  collectives, and the audit trail human+AI co-work needs.
-
 ## Later
 
 - **Matrix:** live validation on NetBSD and DragonFly; one