Type & in the composer to pick a connected machine

[hideoneric]

Problem

To mention another connected machine in the composer, you type @. That menu also searches threads and files, so finding a machine means scrolling past file matches. With a few linked machines this is the slowest way to reach them.

Proposal

A dedicated composer trigger, &, that lists only your machines:

• A bare & lists every machine, and more letters filter by name.
• Picking one inserts the same machine chip @ inserts today. @ keeps offering machines too.
• It reuses the existing machine mention: same items, chip, and gating behind the Linked environments beta, on web and mobile.

& also appears in ordinary prompts, so it stays out of the way: the menu opens only when a machine matches (&, &page=2 stay quiet), && never triggers it, and after a bare & nothing is preselected, so Enter on npm run dev & still sends.

Why &: * was the first idea, but the composer turns *text* into italics. ~ starts home paths, ^ is a dead key on many European keyboards, and ! appears in != and ![image].

Implementation

A working implementation with tests and screenshots is in #30279. Is this direction and scope OK, or would you prefer a different symbol?