buzz-acp: agents ignore DMs unless explicitly @mentioned — mention gate has no DM exemption

## Symptom

In a 1:1 DM channel with a managed agent, the agent does not see or answer plain messages — every single message must explicitly @mention it, even though the DM has exactly two participants. Replying to the agent inside a DM thread behaves the same way: no @, no answer.

Expected: a DM is addressed to the counterparty by definition. Chat platforms don't require @-mentioning your DM partner per message, and the mention requirement is invisible — the agent just looks unresponsive.

## Root cause

The mention gate is a single global flag with no channel-type awareness:

- `crates/buzz-acp/src/config.rs` (`subscription_rules`): `let require_mention = !config.no_mention_filter;` is applied uniformly to **every** target channel — DM channels included. `SubscribeMode::Mentions` is the default the desktop uses for managed agents.
- `crates/buzz-acp/src/filter.rs` (`match_event`): re-checks the `p` tag against the agent pubkey; no mention → dropped.

Notably the harness *does* know the channel is a DM further down the pipeline — `queue.rs` branches on `channel_type == "dm"` for reply anchoring and prompt hints — the DM fact just doesn't participate in the *trigger* decision.

Safety-wise, auto-responding in DMs appears already bounded: DM turns are restricted to the owner and verified siblings (#2591), so exempting DMs from the mention gate would not open agents to arbitrary third-party prompting.

## Propose

Exempt `channel_type == "dm"` channels from `require_mention` (subscription filter and `match_event`), so agents treat any owner message in a 1:1 DM as addressed to them. Group channels keep the existing mention gate unchanged.

Related: #2270 tracks the sibling problem in group-channel threads (agents go deaf in threads they've joined unless re-mentioned). This issue is scoped to DMs only, where the addressed-by-definition argument is strongest.

Observed on a self-hosted relay (deploy/compose derivative, `ghcr.io/block/buzz:main` 2026-07-24, desktop 0.4.24) with desktop-managed agents at default settings.