Desktop: new-DM "To:" search cannot find agents that exist relay-side

# Draft: Desktop new-DM "To:" search cannot find agents that exist relay-side

**Component:** Buzz Desktop (v0.5.3)
**Severity:** UX-blocking for agent workflows (owner cannot start a DM with their own agent)

## Summary
The "To:" user search in the new-DM composer fails to return agent users that verifiably exist on the connected relay. Reproduced independently at three separate agent births on a self-hosted relay (buzz-relay, same version family).

## Repro
1. Create/attest a new agent identity on the relay (NIP-OA attested, profile published; `buzz users get ` returns the profile with display name from the same relay).
2. In Desktop, open a new direct message and type the agent's display name in "To:".
3. Search returns no results, for names that resolve fine relay-side and appear in channel member lists.

## Expected
Any user the relay knows (or at minimum, any member of a channel the searcher shares) should be findable in the DM composer search.

## Workaround
Open the DM from the agent's side (`buzz dms open` as the agent); the human's sidebar then shows the conversation and it works from there — which confirms membership/permissions are fine and only the composer search path is broken.

## Notes
Happy to add relay logs or a specific event trace if useful; this reproduced 3/3 times across different agent identities on the same deployment.

Start by reproducing the new-DM "To:" search in Buzz Desktop against a self-hosted buzz-relay, using an attested agent whose profile is returned by `buzz users get `. Trace the composer search path and compare it with channel member lists and relay-side lookup. Done means an existing relay-side agent appears in the search results and can be selected to start a DM.

Written by the indexing model from the issue text.