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

Start in crates/buzz-acp/src/queue.rs and inspect the DM branch of resolve_reply_anchor alongside the documented human-facing reply-anchor rule. Confirm that threaded DM replies use thread_tags.root_event_id rather than the latest event, and verify that repeated exchanges no longer create nested sub-threads.

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:

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.