fix(cicd): isolate pull request runner jobs

.gitea/runner/README.md

@@ -1,17 +1,18 @@
 # Homelab CI/CD
 
-The native Gitea runner runs on **vps**; production runs on **workstation**.
-Compose, workflow, shell, Python, formatting, YAML, Dockerfile and Kubernetes
-checks appear as separate jobs. Jobs run on `homelab:host`, one at a time; the
-build waits for every check to pass. CI and deploy runs also show a summary with
+The native Gitea runners run on **vps**; production runs on **workstation**.
+Main-branch checks and image builds use `homelab:host`. Pull request and
+non-main checks use `homelab-pr:host` under a separate account without Docker
+access. Each runner accepts one job at a time; the build waits for every check
+to pass. CI and deploy runs also show a summary with
 the release SHA, image build or reuse results, deploy mode, selected services,
 and image digests. Failed runs keep a summary of completed image builds, stage
 results, apply results, and recorded Kubernetes recovery. The final deploy
 summary is in the smoke job; earlier jobs show the state observed at that time.
 Apply success is separate from health and recovery. Update the installed
 workstation controller with `setup-workstation.sh` when no deploy is running.
-No job images or Kubernetes credentials
-are needed on the VPS. Builds use one pinned BuildKit helper container. CI and deploy are separate workflows.
+No job images or Kubernetes credentials are needed on the VPS. Builds use one
+pinned BuildKit helper container. CI and deploy are separate workflows.
 
 ## Runner installation
 
@@ -37,6 +38,32 @@ pushes directly to the registry, and caps retained local cache at 1 GiB with a
 2 GiB free-space target. This is not a hard limit on peak build disk usage.
 Nothing runs `docker system prune`, removes unrelated images, or deletes volumes.
 
+### Pull request runner
+
+Install the unprivileged host runner on the VPS:
+
+```sh
+sudo bash .gitea/runner/setup-pr-runner.sh
+```
+
+Get a registration token from the repository Actions runner settings. Run the
+installer in a terminal. It asks for the token without echoing it, registers the
+runner as `homelab-pr` with label `homelab-pr:host`, then enables the service.
+The work directory is `/var/lib/gitea-pr-runner`. Confirm that Gitea lists
+`homelab-pr:host` before merging the workflow change. An unmatched label can
+fall back to the default job image.
+
+Renovate PR validation uses `pull_request_target`, which reads the workflow from
+the base branch. It checks out the PR head only after runner selection and runs
+that code on `homelab-pr`. Keep this workflow read-only and do not add secrets.
+
+The PR runner has a separate home and tool cache. Do not add it to the `docker`
+group or give it access to `/var/run/docker.sock`. It runs repository code from
+pull requests, so keep its registration and permissions separate from the
+trusted `homelab` runner. This separates users and host permissions, but both
+runners still share the VPS kernel and network. Use a disposable VM if PRs from
+untrusted external authors must be fully isolated.
+
 ## Workstation setup
 
 As the existing SSH deploy user on workstation: