Tasks: minor Slack polish — room-declaration confirmation, ephemeral help, connector signature

Phase 4 of 4. Each task names the requirement it satisfies and the testing-plan row that proves it; the test lands with the change.

[x] A1 — confirm the room from the dispatcher (O1). _apply_channel calls a new _confirm_room(target.ref) after a new declaration; _confirm_room calls self.opener when set and swallows every failure. Req: R1.1–R1.4 · Test: T2 test_a_new_declaration_confirms_the_room_through_the_opener, test_a_failing_confirmation_never_undoes_the_declaration.
[x] A2 — confirm the room from the CLI (O1). manage_channels calls _confirm_room(work_item, config, messages) after an applied add-channel; opens through bus.open_conversation only with a channels section; a failed result is one err line. Req: R1.1, R1.3 · Test: T2 test_add_confirms_the_room_when_channels_are_configured.
[x] B1 — help public (O4). verbs.wants_public_help + PUBLIC_HELP; process_reply answers with bot.say in the member's thread when it holds, the ephemeral otherwise; help_text names the visible form. Req: R2.1–R2.4 · Test: T1 test_help_public_is_read_from_the_first_line_only, T2 test_help_public_answers_as_a_reply_everyone_can_see.
[x] C1 — pin the first-line rule (O5). No production change; the tests above plus test_a_connectors_signature_line_does_not_change_help. Req: R3 · Test: T1, T2.
[x] D1 — docs. docs/capabilities/channels.md (room bullet, grammar bullet, history row); the follow-ups index and file link the issue; evidence files. Deps: A1–C1.