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

Phase 2 of 4. Derives from the approved requirements.md.

Overview

Nothing new is built; two existing seams are called from one more place each.

R1 reuses the conversation opener (issue-317): the dispatcher already holds one (Dispatcher.opener, injected by the daemons over their config getter) and calls it the moment a start is accepted. _apply_channel now calls it once more, right after a new declaration is written, through a small _confirm_room that swallows every failure. The CLI verb (core.workchannels.manage_channels) does the same through bus.open_conversation when the config has a channels section, and reports the outcome as one more line of its output.
R2/R3 reuse the verb parser: parse_verb already hands help its remainder; wants_public_help(verb) reads the first token of the remainder's first line, and process_reply answers with bot.say (an ordinary reply in the member's thread) instead of _tell (the ephemeral).

Why the opener, not a new message (R1)

The ticket offered two shapes: the existing room-opened message at declaration, or a new one-liner. The existing message is the right one: it is the "this room is the conversation" statement, it binds the record (mode: channel) so the first update is delivered as a room message rather than opening the room then, and the open is idempotent by the channel's contract — so declaring, starting and re-declaring in any order produce exactly one message. A conversation already bound in the central channel moves at the declaration instead of at the next event, which is issue-375 R2.2's move brought forward; the pointer left behind is unchanged.

Error handling

Where	Failure	Behaviour
dispatcher	`opener` is `None` (tests, embedders)	nothing opened; declaration and 🎉 as before
dispatcher	opener raises	`logger.exception`, declaration stands, comment settled as executed
CLI	no `channels` section	nothing opened
CLI	bus returns a failed `PostResult`	one `err` line naming the channel and the error; exit code unchanged
inbound	`help public` from a member who may not speak	refused by the unchanged speaker check before this branch

Data & config

No config key, grant, scope, schema, state file or event type changes. One new public name in channels.verbs (wants_public_help, PUBLIC_HELP), one private method on the dispatcher, one private function in core.workchannels.