loopd: job files, run ledger, jobs-invalid

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

docs/runbook.md

@@ -1192,3 +1192,28 @@ delete `EpochEnd`; the next compact continues from it.
 
 **Check.** The next turn on that session either runs on the current epoch, or compact succeeds
 and the next turn's log is `<n>.jsonl` with `epoch` n.
+
+## jobs-invalid
+
+**What you see.** `loopd` does not start. It prints a line naming `{home}/jobs/<name>.toml` and
+this entry:
+
+```text
+loopd: jobs/mail.toml: unknown field `when`, expected `schedule` or `prompt`; see docs/runbook.md#jobs-invalid
+```
+
+**Why.** Job files are owner-written. `loopd` reads them and cannot write them. Unknown keys, a
+name that is not `[a-z0-9-]+`, a name so long that `job-<name>-<yyyymmdd>t<hhmm>` is not a valid
+session id (64 bytes, `[a-z0-9-]`), a schedule that is not five UTC fields, or a missing
+`prompt`, all refuse start so a misspelt job cannot sit silently.
+
+**Confirm.**
+
+```sh
+ls -l "$BOXMAKER_HOME/jobs/"
+```
+
+**Fix.** Rename or edit the named file so it matches the example in `config/jobs/tier.toml.example`.
+Then start `loopd` again.
+
+**Check.** `loopd serve` prints `serving on` and `{home}/jobs/runs.jsonl` exists (empty is fine).