Add channel-owned agent response policy

[mgalpert]

Summary

• add a channel-owned mentions / all agent-response policy to channel metadata and persistence
• make ACP subscriptions refresh live when owners or admins change the policy, while preserving author gates and old-relay fallback behavior
• expose the setting through the CLI and desktop UI, including clear channel indicators

Why

Agent response behavior was runtime-local, so channel owners could not opt selected channels into untagged agent replies without broad per-agent overrides. This makes the channel policy authoritative for every agent member and keeps existing channels mention-only by default.

Impact

• existing and new channels default to mentions
• owners and admins can select all to deliver untagged channel messages to agent members
• running agents refresh their channel subscriptions without a restart
• relays without the new metadata continue using existing local agent rules

Validation

• cargo test -p buzz-sdk update_channel
• cargo test -p buzz-acp agent_response
• cargo test -p buzz-db migration::tests::embedded_migrator_contains_consolidated_initial_schema
• workspace and desktop formatting/clippy checks pass
• desktop frontend: 3,769 tests pass
• desktop Rust: 1,850 tests pass
• desktop production build passes
• mobile format/analyze passes

just ci reaches the mobile test stage but is currently blocked by an unrelated existing assertion in mobile/test/features/channels/channel_detail_page_test.dart (keeps follow mode off while a tall newest message stays visible). This branch does not modify mobile files; the failure reproduces when that test is run alone.