pi compact-tools: let the forum tool draw its own rows

Compacted, a forum send showed only its name; its own renderer shows
the message, folded to 10 lines.

pi forum: give a fresh session its notice with the first run

Appended to a session with no conversation yet -- a new pi, or just
after /new -- the join notice sat ahead of the system message pi records
at the first run, and pi then sent that message without its content: the
provider refused every request with a 400. Queue it for the next turn
instead, which pi places after the system message.

pi forum: fold long messages in the transcript, whole on expand

A message of up to 4000 characters printed in full in both panes; show
its first 10 lines, or 1000 characters, and how much more there is,
as pi does for tool output. The model still gets the whole text.