fix(huddle): load voice instructions into system prompt

[tlongwell-block]

Summary

• promote owner-signed channel huddle instructions into the agent's real system prompt at session creation
• bind the instruction event to the exact channel and verify its Nostr signature before granting system authority
• trim the desktop huddle instructions to six voice-first lines centered on replying immediately when addressed
• preserve legacy adapters through the existing standing-context fallback

Why this seam

This reuses the existing per-channel session/new prompt assembly and the already-posted kind 48106 event. It adds no new desktop-to-harness transport and performs one filtered relay query only when a new channel session is created. Failures are fail-open to no huddle section.

Promoting ordinary channel messages would let arbitrary members inject system-level instructions. This path accepts only a valid event authored by the configured owner with the exact channel h tag.

Testing

• cargo test -p buzz-acp (767 unit + 9 lifecycle tests)
• cargo clippy -p buzz-acp --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• push hook with the repository Hermit toolchain: branch-skew, desktop-check, desktop-typecheck, mobile-test, desktop-test, rust-tests, desktop-tauri-checks all passed

Stacked on #5671.

[tlongwell-block]

Eva review at head 1a2f4d2 (confirmed == PR head; GitHub won't let this account formally approve since the PR was opened under the same login).

• Read the full 3-file diff. The owner-signature + exact-channel-h-tag gate on promoting kind 48106 into the system role is the right security posture: member-authored channel content must not gain system authority. Verify-then-check (signature, author, kind, h tag, non-empty) is complete.
• Ran full cargo test -p buzz-acp at this exact head: 767 + 9 passed, 0 failed. Ran the full desktop Tauri workspace suite at this exact head: 2510 passed, 0 failed (includes the new 6-line prompt pin and the owner/channel rejection tests).
• Fail-open on fetch timeout is correct here: worst case = old behavior, no huddle section.
• Accepted tradeoff, flagged by author: one filtered REST query per new channel-session creation, including non-huddle channels. Bounded by CONTEXT_FETCH_TIMEOUT; fine at this cost.

Verdict: 9/9/9 — ship-shaped. Stacked on #5671; merge after the base.