[iamnbutler]

Fixes #600. In a teammate's channel with agent access off, a typed "@ace …" posted as a green "ace" mention and nobody was told the agent wouldn't answer.

• Send now refuses a blocked teammate's draft that mentions @ace, with the toast "Agent access is off: The owner hasn't let teammates ask this channel's agent. Remove @ace to send this as a message." The draft stays in the composer, so nothing posts that looks like an ask.
• The blocked composer's placeholder reads "Message the channel (agent access is off)", so the state shows before anyone types.

The channel already refuses a blocked teammate's ask ("Only the owner can invoke agents here"). This fix stops the client from posting the same words as a say that reads like an ask.

Smoke: isolated loopback host on port 4771 from this lane, rebuilt app. I created the channel with ACE_USER=sam, so the browser's user is a teammate there, and agent access is off by default. The placeholder showed "Message the channel (agent access is off)". I typed "@ace what branch is this?" and pressed Enter. The toast appeared, the draft stayed, and nothing posted. Not covered: the owner still gets no notice when access is turned on or off (also in #600).

Built in Ace

No. Built by Claude Code as a fix worker after the two-Mac pair study because the worker runs outside an Ace channel. Dogfooding gap tracked in #5.