When replying in a thread with just one agent should not need mention

**Motivation**
What problem does this solve? Who runs into it and when?
Solves the need to do the @agent on every message

**Proposed solution**
Describe what you'd like to see.
When there's just two participants in a thread, me and an agent, the agent should just assume we're talking to it.

**Alternatives considered**
What other approaches did you consider?
none

**Additional context**
Anything else that helps — links, screenshots, prior art.

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Before opening: please [search open issues and PRs](https://github.com/block/buzz/issues?q=is%3Aopen) for duplicates — link the closest one, or say "none found".

Start by tracing how thread replies determine which agent receives a message and where explicit @agent mentions are required. Confirm the behavior for a thread with exactly one agent and one other participant; done means replies in that situation reach the agent without requiring a mention, while existing multi-participant behavior remains unchanged.

Written by the indexing model from the issue text.