docs(tst-30): remove mislabelled macOS instance runner (id 4)

The local act_runner on this Mac registered as instance runner id 4 with
ubuntu-latest/22.04/20.04 labels, so it competed with the two docker
runners on 10.100.0.35 for Linux jobs it had no Docker daemon to run --
13 of the last 20-run window in historiangw landed on it and all but one
failed. Registration deleted; local config kept disabled for re-use with
mac-specific labels.

docs/runbooks/TST-30-second-ci-runner.md

@@ -15,8 +15,13 @@
 > `GET /repos/dohertj2/mxaccessgw/actions/runners` to show ≥2 runners. It does not — it still
 > returns `total_count: 0`, correctly, because both runners are registered at the **instance**
 > level, which is the very condition the "Why" section describes. Use
-> `GET /api/v1/admin/actions/runners` instead (it lists id 1, id 4 — an unrelated local macOS
-> runner — and id 5). For per-job runner attribution, `GET /repos/{owner}/{repo}/actions/runs/{id}/jobs`
+> `GET /api/v1/admin/actions/runners` instead (it now lists only id 1 and id 5; id 4 was removed
+> 2026-08-07 — a local macOS `act_runner` mislabelled `ubuntu-latest`/`ubuntu-22.04`/`ubuntu-20.04`,
+> so it captured Linux-labelled jobs it had no Docker daemon to run and failed them. Its config and
+> registration are kept disabled at `~/gitea-act-runner.disabled-2026-08-07` (launchd plist at
+> `~/Library/LaunchAgents/com.dohertj2.gitea-act-runner.plist.disabled-2026-08-07`) so it can be
+> re-registered with mac-specific labels if a mac-only job ever needs one). For per-job runner
+> attribution, `GET /repos/{owner}/{repo}/actions/runs/{id}/jobs`
 > exposes `runner_id`/`runner_name` on each job; the `actions/tasks` listing does not.
 
 Operator steps to relieve the single shared Gitea Actions runner that CI depends on. The