fix(acp): exempt DM channels from the mention gate; seed mobile presence cache

[tpotu]

Problem

Agents running buzz-acp in mentions mode require a p tag on every message — including in DM channels, at two layers: the relay REQ subscription includes #p, and filter::match_event re-checks the mention locally. The desktop composer auto-adds the agent p tag in agent DMs, but the mobile composer only tags explicit @name mentions. Result: a plain DM sent from mobile never reaches the agent (the relay never delivers it), the agent stays silent, and users conclude the agent is offline.

Separately, the mobile presence cache had no initial fetch (track() was a no-op) — every user and agent read as offline until their next live kind:20001 event arrived, up to 60s for agents publishing heartbeats and indefinitely for users who published before the client connected.

Diagnosed on a live deployment: the same agent answered desktop DMs (p-tagged) within seconds while an untagged mobile DM sat unanswered for two days.

Fix

buzz-acp — DM channels (channel_type == "dm") are exempt from the mention gate; a DM addresses the agent implicitly:

• resolve_channel_filters takes the set of DM channel ids and drops require_mention for them (mentions mode only; All/Config modes unchanged)
• resolve_dynamic_channel_filter takes is_dm; the membership-notification path looks the type up via discover_channels(), fail-closed to mention-required on lookup errors
• filter::match_event takes mention_exempt and skips the p-tag check for DM channels
• setup mode passes the real DM set at startup, an empty set on the dynamic path (no relay probe from setup mode)

mobile — PresenceCacheNotifier.track() now issues a one-shot POST /query (kinds [20001], authors = newly tracked pubkeys); the relay synthesizes presence from its store as relay-signed events with the subject in the p tag, and the handler supports both that shape and live self-signed events. Tracked pubkeys are re-fetched on (re)connect.

Testing

• cargo test -p buzz-acp --lib: 819 passed after merging current main (3 new: DM exemption in static resolution, dynamic resolution, and match_event)
• cargo clippy -p buzz-acp --all-targets: clean
• flutter test (mobile): 1525 passed (2 new: bridge-seeded fetch incl. filter shape, no refetch of already-tracked pubkeys)
• flutter analyze: no issues
• E2E on a live relay: DM without any p tag → agent responds within one turn; group channels still require @mention

🤖 Generated with Claude Code

[Giszmo]

#4201 is the more concise and consistent fix for this, isn't it?