feat(ui): mount the WC shell on WorkUnitClient — transcript, queue, composer, model, one mount path

[trieloff]

Follow-up to #2274 (part of #1666). The client protocol landed with both adapters, one conformance suite, and the tab strip rendering from toTabDescriptors on leader and follower. The rest of the shell still has two wirings.

What is left

Surface	Leader today	Follower today	After
transcript	requestScoopMessages → onScoopMessagesReplaced → controller.loadMessages	onSnapshot → controller.loadMessages	client.snapshot(id) / subscribe(id)
selection	setSelectedScoopJid + replay	sync.selectScoop + onSnapshot	client.snapshot(id) (selection IS the call)
queued pile	queuedIds on the replay envelope (#2362)	not sent	snapshot.queuedIds, absent ≠ empty
composer send	AgentHandle.sendMessage (implicit selection)	sync.sendMessage (implicit selection)	client.send(id, input)
stop	stopScoop(jid)	sync.stop()	client.signal(id, 'stop')
model pill	modelFor(record) + set-scoop-model	wc-follower-model-surface + model.select	summary.model as the single read
mount	mountWcUiLive / mountWcUiExtension	mountWcUiFollower	one mount path taking a WorkUnitClient

Why it was not in #2274

attachWcClient binds OffscreenClient to the VFS, the terminal, the monitor, sprinkles and the permissions surface as well as the chat — none of which the protocol covers or should. Collapsing the mounts means deciding what those get instead, which is a design question of its own (and overlaps #2276's CapabilityBroker). Shipping the protocol with the strip cut over keeps the risk of each step legible.

Done when

• The transcript, the queued pile, the composer and the model pill read from WorkUnitClient on both sides.
• A tab click goes through client.snapshot(id) rather than two transport-specific selection calls.
• mountWcUiLive / mountWcUiFollower / mountWcUiExtension share one mount that takes a client.
• The conformance suite grows the cases the new surfaces need (queue reconcile parity, send/steer, stop mid-turn).
• No follower feature needs a parallel UI state machine (review-patterns category 8).

Legacy path deletion stays with #2365.

[github-actions]

This one tastes faintly of race condition and regret.

[github-actions]

^{🎫 Backlog Dispatcher}

Not dispatching this one. This is a cross-cutting redesign collapsing multiple mount paths and touching the permissions/security surface, and the body itself calls the missing behaviour “a design question of its own” — not one small contained PR.

Nothing was changed and no pull request was opened. This decision is made once — remove the backlog-skipped label to put the issue back in the queue.

[augmentcode]

^{Backlog Dispatcher🎫}

Skipped by Backlog Dispatcher. Reason: Broad leader/follower protocol and UI refactor rather than a small localized change.

[augmentcode]

^{Backlog Dispatcher🎫}

Skipped by Backlog Dispatcher. Reason: Large architectural cutover across transcript, selection, queue, composer, model, and mount paths.