buzz-acp: DM replies anchor to the latest message instead of the thread root — every exchange nests one level deeper

## Symptom

In a 1:1 DM with a managed agent, once a thread exists, every agent reply anchors to the human's **latest comment** rather than the thread root. Each exchange therefore nests one level deeper — the user has to click-expand a sub-thread of a sub-thread of a sub-thread to follow a simple back-and-forth conversation with their own agent.

## Root cause

`crates/buzz-acp/src/queue.rs` — the human-aware reply-anchor logic documents the right intent directly above the code:

> Human-facing turns are anchored so replies stay readable at layer 1:
> - in a thread → anchor to the thread ROOT (no depth-2 nesting)

…and `resolve_reply_anchor` implements exactly that for group channels. But the DM branch contradicts it:

```rust
let reply_anchor = if is_dm {
thread_tags
.root_event_id
.is_some()
.then(|| last_event.event.id.to_hex()) // ← anchors to the LATEST message, not the root
} else {
resolve_reply_anchor(...) // ← group channels: anchors to ROOT
};
```

When the triggering DM message is inside a thread, the anchor is `last_event.event.id` — the user's newest comment — so the agent's reply becomes a child of a child, and the nesting compounds every round.

## Propose

In DMs, match the documented layer-1 rule: anchor to `thread_tags.root_event_id` when inside a thread (same as the group-channel human path). Worth discussing whether top-level DM exchanges should thread at all — a 1:1 DM arguably reads best flat, with threading reserved for explicit user-initiated threads — but the root-anchoring fix alone eliminates the compounding sub-chat nesting.

Observed on a self-hosted relay (`ghcr.io/block/buzz:main` 2026-07-24, desktop 0.4.24), desktop-managed agents, default settings. Sibling issues: #2270 (thread deafness without re-mention), and the DM mention-gate issue filed alongside this one.