Add Forgejo Actions runner substrate and status probe

Documents interim coulombcore org runner for forgejo.coulomb.social and
adds make forgejo-runner-status for operator health checks.

docs/forgejo-actions-runner-substrate.md

@@ -0,0 +1,76 @@
+# Forgejo Actions Runner Substrate
+
+Last reviewed: 2026-07-03
+
+Status: first supported runner on coulombcore (interim). Forgejo instance runs on
+railiance01; runner host is intentionally separate per Forgejo security guidance.
+
+## Purpose
+
+Provide a forge-owned Actions runner for `https://forgejo.coulomb.social` so
+repos in the `coulomb` organization can build and publish without workstation
+involvement. Mirrors the Gitea runner contract in
+`docs/gitea-actions-runner-substrate.md`.
+
+## First Supported Runner
+
+| Field | Value |
+| --- | --- |
+| Runner name | `railiance-coulombcore-build-01` |
+| Runner scope | `coulomb` organization |
+| Host | coulombcore (`92.205.130.254`) |
+| Instance | `https://forgejo.coulomb.social/` |
+| Binary | `forgejo-runner` v6.3.1 |
+| Runtime | Host mode (Docker not installed on host yet) |
+| Cluster deploy authority | Not included |
+
+### Labels
+
+```text
+self-hosted:host,linux:host,linux_amd64:host,container-build:host,registry-publish:host,railiance01:host
+```
+
+`self-hosted` preserves compatibility with existing Gitea workflow `runs-on`
+values during migration. Container image builds require Docker on the runner
+host — install before cutover of image-publishing workflows.
+
+## Service Layout
+
+| Path | Purpose |
+| --- | --- |
+| `/usr/local/bin/forgejo-runner` | Runner binary |
+| `/var/lib/forgejo-runner/config.yaml` | Generated config |
+| `/var/lib/forgejo-runner/.runner` | Registration state (secret) |
+| `/etc/systemd/system/forgejo-runner.service` | systemd unit |
+
+## Probe Evidence (2026-07-03)
+
+- Org `coulomb` created on Forgejo.
+- Runner registered and `systemctl is-active forgejo-runner` → `active`.
+- Repo `coulomb/forgejo-actions-probe` workflow `probe.yaml` (`runs-on: self-hosted`)
+  completed with `status: success` (API: `/api/v1/repos/coulomb/forgejo-actions-probe/actions/tasks`).
+
+## Operator Commands
+
+```bash
+# Runner health (from railiance-forge)
+RUNNER_HOST=coulombcore make forgejo-runner-status
+
+# Forgejo app smoke (from railiance-apps)
+make -C ~/railiance-apps forgejo-smoke
+
+# Generate a new org registration token (in cluster, no token in logs)
+KUBECONFIG=~/.kube/config-hosteurope kubectl exec -n forgejo deploy/forgejo-gitea -- \
+  forgejo actions generate-runner-token --scope coulomb
+```
+
+## Secret Boundaries
+
+Same as Gitea runner substrate: registration tokens and `.runner` files stay
+off Git. Use approved secret paths on the runner host only.
+
+## Follow-On
+
+1. Install Docker on the approved runner host for `container-build` jobs.
+2. Move runner to a non-legacy host before coulombcore decommission (T03 drain).
+3. Add repo-scoped registry credentials in Forgejo org secrets for image push.