ACP agent misses plain room messages when --channels is used without --subscribe

## Bug
The VECTOR Buzz ACP service did not receive ordinary messages sent in its room unless they explicitly mentioned `@vector`.

## Evidence
- Service command was `buzz-acp ... --channels 7c5d7808-7d93-4124-9e3b-42a760132b40` with no `--subscribe` flag.
- Startup log reported `subscribe=Mentions`.
- ACP `Mentions` mode sets `require_mention=true`; relay filtering requires a `p` tag for the agent pubkey.
- Room history contained ordinary user messages with no `@vector` mention and no corresponding agent response.

## Impact
Agents configured only with `--channels` silently miss room messages, contradicting the expected room behavior.

## Workaround applied locally
The VECTOR service is now configured with `--subscribe all --channels ...` and restarted successfully. The service reports `subscribe=All` and is connected.

## Suggested fix
Clarify/adjust the default for `--channels` (or service templates) so a channel-scoped agent receives room messages by default, or make the mention-only behavior explicit and fail/warn loudly when no `--subscribe` mode is set. Add an integration test covering a plain top-level channel message without a mention.