Hide @ace from collaborators when agent access is off #498

Dogfooding: when the owner has Collaborator agent access off, a collaborator's composer still offers @ace: the Ace mode (placeholder "Ask the agent"), the @ace mention suggestion, and Ace mode as the default. Sending then shows a refusal ("Switch to Chat" / "Remove @ace").

Expected: a collaborator who can't invoke the agent sees no @ace at all. The composer is in Chat mode with "Message the channel", no Ace mode toggle, and no @ace mention suggestion. If access is turned on, @ace appears again.

Today this is deliberate: apps/app/src/composer.tsx uses BLOCKED_MODES, commented "Teammates keep the Ace mode and their drafts while the owner has turned invocation off", and useInvocation refuses on send. Changing it means dropping Ace mode and the mention list when shared === false && !owner, and deciding what happens to a saved Ace-mode draft (send as a message with the mention stripped, or keep it until access returns).

Related: #497 (collaborators can stop runs with access off). Dogfooding: #5