Typing @ and part of a machine's name in the composer now offers the user's
other T3 Code environments beside threads and files, each with its machine
icon. Picking one inserts a chip. The message carries only the machine's
identity (environmentId and label, never an address), and the agent is told
to link it with t3_environment_link if it is not linked yet, then pass the
environmentId to delegate_task, t3_thread_launch and the other tools.

Web keeps mentioned machines in the draft beside attached threads, persists
them, restores them on undo and failed sends, and renders the chip in the
composer and in sent messages. Mobile offers them in the @ popover and draws
the chip natively.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>