[Bug] buzz-acp agents go deaf in threads they've joined unless re-mentioned.

> Issue generated by Kimi K3

**Describe the bug**

The buzz-acp agent harness only responds to messages that explicitly @mention the agent. Once an agent has joined a thread, follow-up replies in that thread are silently ignored unless the human re-tags the agent on every single message.

The gate exists at two levels:
1. Relay subscription — `SubscribeMode::Mentions` (the default) sets `require_mention: true` on channel filters, so the relay never even delivers untagged thread replies to the harness.
2. Rule matching — `match_event` in the filter module re-checks for the agent's `p` tag; no match → event dropped.

**To Reproduce:**

1. Start a buzz-acp agent with default config (Mentions subscribe mode) joined to a channel.
2. In that channel, send a message @mentioning the agent — it responds in a thread.
3. Reply in that same thread without tagging the agent (e.g. "yes, go ahead").
4. The agent never sees the reply — no reaction, no response. Re-tag the agent in the same thread and it responds immediately.

**Expected behavior:**

The agent should follow threads it is participating in — like Slack/Discord thread notifications — so a human can continue the conversation naturally without re-mentioning on every reply. Agents must never auto-reply to each other without an explicit mention (anti-loop invariant).

**Agreed solution direction (participation-based thread following):**

- Track per-channel "participating thread roots" in the harness — seeded when the agent is mentioned in or replies to a thread (NIP-10 `root_event_id` via the existing thread-tag parser), with TTL/LRU expiry so stale threads don't wake the agent forever.
- Widen the relay subscription in Mentions mode (drop relay-side `require_mention` for chat message kinds) and move gating fully into rule matching: queue if `mentioned(agent)` OR `root_event_id ∈ participating[channel]`.
- Anti-loop invariant: participation-follow applies to human-authored replies only; agent-authored replies still require an explicit mention. Never queue the agent's own events. The existing author gate, per-channel in-flight dedup, and fail-closed filter semantics stay unchanged.
- Config: new per-rule fields `follow_participating_threads` (default true in Mentions mode) and `follow_thread_authors = "humans" | "all"`.
- Scope: `buzz-acp` only — no relay changes, no new event kinds, no protocol changes.

**Supporting Material**

- The mention gate: `match_event` stage 3 (`require_mention`) in `crates/buzz-acp/src/filter.rs`; channel filter construction in `crates/buzz-acp/src/config.rs` (`SubscribeMode::Mentions` sets `require_mention: !config.no_mention_filter`).
- The thread-tag parser already exists (`parse_thread_tags` in the queue module) and yields `root_event_id` / `parent_event_id` — the seed data for participation tracking.
- Existing loop-breakers that must be preserved: author gate (`respond_to` owner/allowlist/sibling modes), one in-flight prompt per channel with batched queueing, fail-closed evalexpr filter semantics, relay-side exclusion of workflow execution kinds from triggers.

**Environment**

- Component: `buzz-acp` agent harness (Rust)
- Affects: all deployments using the default Mentions subscribe mode

**Additional context**

Repo conventions: read `AGENTS.md` first (Nostr-first patterns, kind registry in `buzz-core`, `just ci` quality gates). Existing unit tests live alongside the filter/queue modules; relay-level E2E lives in the integration test client crate.

**User story seed:** As a Buzz user, I want the agent to keep responding in a thread it has joined without me re-tagging it on every message, so conversation flows naturally — while agents never auto-reply to each other without a mention.