Summon the agent with @ai

Shorter to type, and it says what it is rather than what the prototype is
called — the room is talking to an assistant, not to chopin.

A shorter mention has more prose to collide with, so the boundaries earn more
scrutiny than they did before. The leading guard rejects a longer token, so
`hi@ai.dev` is an address rather than a summons, and the trailing boundary
rejects `@aiden` and `@airflow`. All three are now cases in the test.

Verified live: four messages containing `@airflow`, `@aiden` and an email
address started no turn and all reached the transcript; `@ai` then started one.