loopd: heartbeat tick on the background slot

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

docs/runbook.md

@@ -1217,3 +1217,29 @@ ls -l "$BOXMAKER_HOME/jobs/"
 Then start `loopd` again.
 
 **Check.** `loopd serve` prints `serving on` and `{home}/jobs/runs.jsonl` exists (empty is fine).
+
+## heartbeat-prompt-unreadable
+
+**What you see.** `loopd` is serving, but heartbeat turns do not run. It logs this line once,
+until the file reads:
+
+```text
+loopd: heartbeat.md: Permission denied (os error 13); see docs/runbook.md#heartbeat-prompt-unreadable
+```
+
+**Why.** `[heartbeat] prompt` is next to the config file (default `heartbeat.md`). The scheduler
+skips the tick rather than send an empty or guessed user message. A missing file is the same:
+restore the starter from `config/heartbeat.md` in the repository.
+
+**Confirm.**
+
+```sh
+ls -l "$(dirname "$LOOPD_CONFIG")/heartbeat.md"
+```
+
+**Fix.** Restore mode `0644` (or whatever the owner uses for the other prompts) and content.
+The next tick after a successful read logs nothing extra and runs the standing `heartbeat`
+session.
+
+**Check.** `sessions/heartbeat/0.jsonl` gains a new `User` whose text starts with `It is ` and an
+RFC 3339 timestamp.