fix(desktop): route same-owner remote agent mentions

[dafacto]

Summary

• make a same-owner remote agent mentionable from a stream or forum channel where that exact agent pubkey is already a member
• route the existing remote pubkey without creating, starting, or adding a local managed-agent instance
• preserve strict boundaries for DMs, community search, other-owned agents, and relay agents configured with respond_to: "nobody"
• tolerate future relay response-mode strings without dropping the entire relay-agent directory

Root cause

The composer allow-list treated the local managed-agent store as the primary source of agent identity. On another Buzz client, a channel could contain an agent owned and hosted by the same user, but that client had no local managed record for the pubkey. The remote member was therefore filtered from autocomplete or could be confused with a local instance lifecycle.

This change treats verified NIP-OA ownership plus active channel membership as the remote routing signal, scoped only to channel composers. Admission waits for a successful relay-directory response so an explicit heartbeat-only policy cannot be missed during startup or directory errors.

User impact

A user can mention an agent hosted by another Buzz client in a shared stream or forum. Buzz sends the mention to the exact existing pubkey; local managed-agent setup remains untouched. Active Huddles reuse the existing remote-pubkey synchronization path.

Fixes #3277.

Validation

• Claude peer review: LGTM WITH NITS after independent TypeScript, Biome, Rust, unit, and focused browser verification
• desktop unit suite: 4,538 passed
• desktop check, typecheck, file-size/style guards, and production/e2e builds passed
• complete mentions smoke suite: 62 passed in the full run; its one pre-existing typing-space flake passed immediately in isolation (63/63 scenarios green)
• focused Rust response-mode and future-mode compatibility tests passed; cargo fmt --check passed

Scope

This deliberately does not broaden direct-message or community-search agent discovery, and it does not make a remote identity a local managed instance.