No UI surface to inspect relay-agent (kind:10100) directory records

**Summary.** `useRelayAgentsQuery` / `list_relay_agents` (kind:10100) feed ~8 surfaces — mention autocomplete, members sidebar, new-message recipients, tray menu, Pulse — but there is no UI that lists the relay directory records themselves. When a record is minimal or malformed (see #5869), the failure is opaque: agents silently never appear, and nothing shows the operator what the directory actually contains.

Also, when a 10100's content lacks `name`, the fallback label is the agent's full npub (`agents_from_events` in `desktop/src-tauri/src/nostr_convert.rs`), which is unusable in mention search and reads as noise in the surfaces that do render it.

**Suggested fix.** An "Agents on this relay" panel (read-only is enough) showing each record's pubkey, name, `channel_ids`, `respond_to`, and — ideally — why the agent is or isn't currently mentionable in a given channel. This would have turned a multi-hour diagnosis into a glance.

Related: #5869.