Add end-to-end guarantee that agent event results are posted back to the source room

## Bug / feature gap
When an agent is asked to post event/task results back into a Buzz room, the result can be absent from the room because the agent never receives the originating plain room message under the default mention-only configuration.

## Observed behavior
The VECTOR service was configured with `--channels ` but no `--subscribe` flag, started in `subscribe=Mentions`, and therefore only accepted events containing a `p` tag for the agent. A normal room request did not trigger processing, so no result was posted back.

## Impact
Requests made in a room can appear to be ignored, and users cannot rely on event results being posted in the same room.

## Suggested fix
Add an end-to-end test: publish a plain room request, process it, and assert a result/reply event appears in that same channel. Also document/enforce the expected room delivery mode and preserve the source channel when posting results.

Start by tracing the VECTOR service's room subscription configuration, plain room event processing, and result publishing path. Add an end-to-end case that publishes a plain room request and verifies a result or reply event in the same channel; confirm the expected delivery mode is documented or enforced.

Written by the indexing model from the issue text.