fix(issue-42): review R1 - stop's exit 1 also comes after the stop

The registry update after the kill chain raises SystemExit(1) when the row
disappeared during the run (stop_session.sh:310), so "exit 1 changed nothing"
was false on that path. The stop table now says so, the code is declared as a
passthrough, and the test requires every integer SystemExit in an entry
script's heredocs to be declared - the check that would have caught it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.agents/skills/multi-agent-mux-stop/SKILL.md

@@ -59,13 +59,13 @@ bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh \
 | Exit | Meaning |
 |---|---|
 | 0 | Stopped; or already `stopped` (no-op) |
-| 1 | YAML not found, or the session is not registered |
+| 1 | From the checks before the stop, nothing was changed: YAML not found, or the session is not registered. **The registry update after the stop also exits 1** (the row disappeared from the YAML during the run) |
 | 2 | Bad arguments (including the retired `--mode` / `--capture-id` / `--graceful`), or `--agent` omitted and not resolvable |
 | 3 | `--purge-conversation` without `--yes` |
 | 4 | Purge aborted: the herdr session survived the kill chain |
 | other | The registry update after the stop (YAML / DB write) failed and its code is passed through |
 
-Exits 1, 2 and 3 come from checks before anything is stopped, so nothing was changed. On an `other` exit the session may already be down while its row still says `running`: check with `multi-agent-mux-status`, then run the stop again (it is idempotent).
+Exits 2 and 3, and exit 1 from the pre-stop checks, come before anything is stopped, so nothing was changed. On exit 1 from the registry update, or an `other` exit, the session may already be down while its row still says `running`: check with `multi-agent-mux-status`, then run the stop again (it is idempotent).
 
 **Idempotency**: if the row is already `status: stopped`, the script prints `already stopped (...)` and exits 0 — re-running is a safe no-op.