Mentioning an agent from another machine never wakes it — agent start on mention is sender-side only

### Setup

- Machine A ("host", always on): runs Buzz Desktop and manages/hosts all agents.
- Machine B (chat client): same user, same relay, no locally managed agents.
- All agents are `role=bot` channel members on the relay. Desktop v0.5.4.

### What happens

Mentioning an agent from machine B never starts it on machine A. Only agents that are already running on A (e.g. via start-on-app-launch) respond; a managed-but-stopped agent stays silent with no error anywhere.

### Why (as far as I can tell from the source)

Waking an agent on mention is implemented **sender-side only**: `desktop/src/features/messages/ui/useMentionSendFlow.ts` resolves mentioned pubkeys against the *sender's* managed-agents list and calls the start mutation for those. I could not find any host-side path (in `desktop/src-tauri/src/managed_agents/` or the crates) that starts a stopped managed agent when a mention for it arrives from the relay. So in any split topology — the same one as #3609 — mentions from other machines only reach agents that happen to be warm.

This also bites subtly on a single machine: an agent with start-on-launch disabled answers when mentioned locally (sender-side start kicks in), which makes the multi-machine silence look random.

### Workaround

Enable start-on-app-launch for every agent on the hosting machine.

### Suggestion

Have the hosting app subscribe to mentions of its own managed agents and start them on demand — that makes wake behavior symmetric with local sends. If that's out of scope, documenting start-on-launch as required for multi-machine setups would already help.

Related: #3609 (mention picker filtered to locally-hosted agents — same split-hosting topology).

Start with desktop/src/features/messages/ui/useMentionSendFlow.ts to trace the existing sender-side wake path, then inspect desktop/src-tauri/src/managed_agents/ and the related crates for host-side mention handling. Reproduce the split-host setup described in the issue and trace relay-delivered mentions. Done means a stopped managed agent on the hosting machine wakes when mentioned from another machine, or the required limitation is documented.

Written by the indexing model from the issue text.