Make one-to-one agent DMs implicitly addressed

[mdeiters]

Summary

• Treat verified one-to-one DMs containing the running agent as implicitly addressed.
• Keep group DMs mention-gated by distinguishing recipient p tags from explicit mention references.
• Carry DM participant metadata through startup and dynamic channel resolution, including setup mode.

Buzz adds recipient p tags to every DM participant for delivery and notifications. The ACP harness previously treated those tags as explicit mentions, so it could not safely distinguish a one-to-one agent DM from a group DM. This change uses signed channel participant metadata and explicit mention-reference tags to make that distinction.

Missing or malformed channel metadata fails closed and keeps mention filtering enabled.

Related issue

None found.

Testing

• cargo test -p buzz-acp (620 unit tests, 9 integration tests)
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo fmt --all -- --check
• just ci