fix(acp): answer unmentioned direct messages

[arisgysel-design]

Summary

• remove the relay-level #p mention filter for confirmed DM channels at startup and on dynamic joins
• bypass the ACP rule-level mention gate only for confirmed DMs
• preserve mention requirements for streams, unresolved channel types, and setup mode

Safety

DM author authorization remains unchanged: only the owner or verified siblings can trigger the agent. Unknown channel types continue to require a mention for rule matching.

Tests

• cargo test -p buzz-acp
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• just ci

Closes #2747

[svndco]

Independent review — blocking behavior concern

I checked this out at 022b0bc2ac8e6397fd85848dffe5890c5c129a3f and verified:

• cargo test -p buzz-acp: 595 library + 9 integration tests passed
• cargo clippy -p buzz-acp --all-targets -- -D warnings: passed
• cargo fmt --all -- --check: passed

The relay-side subscription widening is necessary, but the in-process exemption is currently keyed only to is_confirmed_dm. The DM author gate intentionally accepts both the owner and verified same-owner siblings, so an unmentioned sibling-agent event in a DM will now match require_mention rules. That can reintroduce agent-to-agent wake loops; #2777 correctly narrows the exemption to is_dm && author == owner.

I consider that scope issue blocking. Please compute the mention exemption from both confirmed-DM status and owner authorship, while continuing to let the broader subscription deliver events for in-process filtering. Add a regression test showing an unmentioned sibling-agent DM does not match.

[arisgysel-design]

Addressed the blocking review in 060c72e6: the relay subscription still delivers all confirmed-DM events, but the in-process mention exemption is now scoped to confirmed DM && author == configured owner. Added regressions proving an unmentioned owner DM matches while an unmentioned sibling-agent DM does not. Full just ci passes.