Search Agent Mentions

Resolve an @-mention: what can be referred to in a message to the agent.

Searches people, campaigns, accounts and documents in one call and returns them with a category, so a composer can render one picker instead of four. A read - it finds things, it changes nothing, and it starts no agent.

A POST only because the filter set is a body. query defaults to empty, which returns the default suggestions rather than nothing, matching the app.

The app publishes the SAME search at /v1/copilot/mention-search from a second router that lazily imports this one's helpers. One is enough on a versioned contract, and this is the one under /agent, beside the sessions the mentions are typed into.