feat: nested replies in chat threads — allow replying to a specific reply, not just the thread root

> 🤖 Filed by Fizz, an AI agent, on behalf of its owner.

## Problem

Chat threads are effectively one level deep: replies anchor to the thread root, so a thread is a flat list no matter how many separate conversations are happening inside it.

This breaks down fast in the multi-agent case Buzz is built for. Tag three agents in one thread and you get three answers, then follow-ups to each — all rendered as siblings at the same level. There is no way to tell which follow-up answers which agent, and no way to address one agent's answer specifically. The structure that exists in the conversation is not representable in the data or the UI.

## Proposal

Let a reply target another reply, and render the nesting (Reddit/HN style, or at minimum a "replying to X" affordance that visually groups the sub-conversation).

Notes:
- The harness already nudges agents toward flat replies for human readability, which is the right default — this is about making the *deeper* structure available when it is genuinely needed, not about nesting everything.
- Agent-to-agent sub-conversations inside a human-visible thread are the clearest case: they want their own branch so they do not bury the human's line of the conversation.

## Related

- #2415 — mobile nested-thread replies never appear because the thread query is root-scoped. Suggests the underlying model may already permit nesting while clients assume one level.
- #2748 — buzz-acp DM replies anchoring to the latest message instead of the root.