@ace from a teammate without agent access posts as a mention and is silently ignored #600

When a channel's agent access is off, a teammate can still send "@ace …". It posts as a green "ace" chip, like a real agent mention, and nothing runs. Neither side is told.

What was seen

• Guest (A) in Sam's b-chan, access off: typing "@" offered only "samechocolixir", so A typed "@ace what branch is this repo on?" as plain text and sent it. It posted as "iamnbutler — [ace chip] what branch is this repo on?", the same style as an earlier run that did work. No reply, no "agent access is off" notice, no toast. Tokens and cost didn't change.
• Owner (B): the message just sits there unanswered; "My view shows no note that it was blocked or ignored."
• Turning agent access on or off posts nothing in the channel. A guest finds out only from the composer changing or by hovering the 12 px icon.

The map expects "a typed @ace is plain text" and the channel to refuse with Only the owner can invoke agents here (^588).

Steps

1. Owner turns agent access off in a channel; a teammate on another Mac opens it.
2. The teammate types "@ace what branch is this repo on?" by hand and sends it.

Expected: the text stays plain, or the send says agent access is off, and the owner sees that a request was declined. Related: #498.

Map: ^588, ^432.

Found in the 2026-10-22 two-Mac pair study.