buzz-acp: heartbeat feed mentions have no thread context — responses go unthreaded or silent

## Summary

Agents processing @mentions during **heartbeat turns** have no thread context and send unthreaded replies (or no replies at all), even though the triggering mention was in a thread. This is a distinct issue from #3825/#3908, which fixed the **normal dispatch path**.

## Root Cause

`dispatch_heartbeat` (`lib.rs:3923-3956`) calls `default_heartbeat_prompt` (`lib.rs:4012-4028`) with `batch=None`. The prompt instructs the agent to call `buzz feed get --types mentions`, but gives **no `--reply-to` instruction** for how to reply to those mentions.

The normal dispatch path injects `IMPORTANT: For ordinary replies in this turn, use --reply-to ` via `append_reply_instruction` (`queue.rs:1149-1156`), but that function is **never called** in the heartbeat path.

**Result:** When an agent finds an @mention in `buzz feed get` during a heartbeat, it has no thread root and either:
- Sends a reply without `--reply-to` (creating a new top-level message instead of threading), or
- Sends no reply at all

## Observed Symptom

Users see agent activity in the observer/activity feed (kind:24200 encrypted frames) showing the agent processed a turn, but **no channel message** appears in the thread.

Confirmed in channel COMPDE-454 — Osmosmith messages appeared in the activity feed but not in the thread for several mentions processed during heartbeat turns.

## Suggested Fix

Extend `default_heartbeat_prompt` (`lib.rs:4012-4028`) to explicitly instruct:

> For each actionable feed @mention:
> 1. Call `buzz messages thread --channel --event ` to resolve the thread root
> 2. Send your reply using `--reply-to `

**Do NOT pre-resolve in the harness** — the harness does not know which feed items the agent will retrieve or act on. The instruction must be in the prompt.

If custom persona `heartbeat_prompt` overrides exist, the durable rule should also live in the shared base prompt to cover all heartbeat turns.

## Test Gap

- No test exercises the heartbeat path where an agent replies to a feed-sourced @mention and verifies `--reply-to` is present or instructed
- `test_format_prompt_single` tests normal dispatch but not the heartbeat code path

## PR Overlap Check

- **PR #3908** fixes top-level vs threaded replies in the **normal dispatch path** (`queue.rs`). It does NOT touch the heartbeat path.
- **PR #3183** propagates automatic reply context for normal batches but **explicitly clears it** for heartbeat turns. It does NOT fix feed-driven heartbeat replies.

## Affected Code

| File | Lines | Issue |
|------|-------|-------|
| `crates/buzz-acp/src/lib.rs` | 3923-3956 | `dispatch_heartbeat` — passes `batch=None`, no thread context injected |
| `crates/buzz-acp/src/lib.rs` | 4012-4028 | `default_heartbeat_prompt` — no `--reply-to` hint for feed-driven replies |
| `crates/buzz-acp/src/queue.rs` | 1149-1156 | `append_reply_instruction` only fires on normal dispatch, not heartbeat |

## Confidence: High

Confirmed against `origin/main@07a3c768` (2026-08-10) by Grumplestiltzkin.

## Related Issues

- #3825 — "Agent reply visible in Activity log but not displayed in channel/thread" (symptom)
- #3908 — Fixes the **normal dispatch path**, not the heartbeat path

## Research

Full investigation: `RESEARCH/AGT_01_AGENT_ACTIVITY_NOT_THREAD.md` in the Buzz workspace

Start in crates/buzz-acp/src/lib.rs at dispatch_heartbeat and default_heartbeat_prompt, then compare the normal dispatch behavior in queue.rs at append_reply_instruction. Update the heartbeat guidance for feed-sourced mentions and add coverage for this heartbeat path alongside test_format_prompt_single. Done means actionable mentions can resolve their thread and replies include the appropriate --reply-to context.

Written by the indexing model from the issue text.