Add chat_join() and invites on chat_poll()

[TroyHernandez]

Phase 1d — the change that lets corteza stop reading chat_poll()$raw.

The inviter is the point

chat_poll()$invites returns chat_invite() records carrying the channel and who issued the invitation.

That second field is why this PR needed cornball-ai/mx.client#21 first. corteza gates invites on an operator list and refuses everything else; a record with only a room id would have left it walking $raw to find the sender, and 1d would have achieved nothing.

inviter is NA when the transport does not say, never NULL. A consumer deciding whether to accept has to tell "someone I do not trust" from "I could not tell who" — both refuse, for different reasons, and only one is worth logging as a rejection. NULL would collapse the second into the absence of a field.

No timestamp

The one place this record differs from chat_message() and chat_reaction(). An invitation is a standing state rather than an event at a moment, and Matrix's stripped invite state carries no reliable origin_server_ts. A field that could only ever be NA is worse than no field, so there isn't one — asserted, so it does not quietly acquire one later.

chat_join()

Matrix goes through mx.api::mx_room_join(), Slack through conversations.join. Errors propagate, same reasoning as chat_react(): a join that silently did nothing leaves the caller believing it is in a room it will never hear a word from, which looks exactly like an idle room.

The capability split earns itself again

adapter	invites	join
Matrix	tracks installed mx.client	TRUE
Slack	FALSE	TRUE
IRC, loopback	FALSE	FALSE

Slack is FALSE/TRUE: a bot is added to a private channel by a member, which arrives as no event this adapter can see, but it can join an open channel itself. A single flag would have a consumer either wait for invitations that never arrive or refuse to join a channel it could.

Testing

660 assertions, 298 on a bare runner with no mx.* installed. Four mutations, all caught: dropping $invites entirely, dropping the inviter from the record, letting a NULL inviter through as NULL, and Slack claiming it receives invitations.

R CMD check --as-cran clean but for the expected new-submission NOTE. mx.client floor raised to 0.2.0.4.

Downstream

corteza drops matrix_invite_inviters() and its last $raw read, at which point grep -c 'res\$raw' R/matrix.R reaches zero — the migration plan's stated finish line for phase 2.