feat(github-runner): harden Linux runner fleet

apps/github-runner/README.md

@@ -0,0 +1,61 @@
+# GitHub Runner Fleet
+
+ArgoCD owns `apps/github-runner/github-runner.yaml`. Do not patch live runner
+Deployments with `kubectl`; update this manifest and let ArgoCD reconcile.
+
+## Runner Shape
+
+All repo-scoped Linux runners use:
+
+- `ACCESS_TOKEN` from the `github-runner-token` Secret
+- `RUN_AS_ROOT=false`
+- `EPHEMERAL=true`
+- `LABELS=self-hosted,linux,fc-build-linux`
+- writable non-root paths under `/home/runner` for .NET, NuGet, XDG cache, and
+  Actions tool cache
+
+`github-runner` for `FlowerCore.Common` is single-replica because it retains the
+original Longhorn ReadWriteOnce NuGet PVC. `github-runner-sharedpos` and the top
+Linux-cost repo runners use two replicas with per-pod `emptyDir` caches. That is
+the safe backlog-drain strategy: no two pods share one RWO PVC.
+
+## Post-Merge Proof
+
+After the PR is merged and ArgoCD syncs, verify the runner fleet:
+
+```bash
+kubectl -n github-runner get deploy,pods,pvc
+```
+
+Verify GitHub registration for the repo-scoped runners:
+
+```bash
+for repo in FlowerCore.Common FlowerCore.Shared.Pos FlowerCore.Puppet FlowerCore.Signage \
+            FlowerCore.DMS FlowerCore.Telephony FlowerCore.Print.Web FlowerCore.Chat \
+            FlowerCore.MySQL FlowerCore.Kiosk.Linux; do
+  echo "=== $repo ==="
+  gh api "/repos/astoltz/$repo/actions/runners" \
+    --jq '.runners[] | select(.labels[].name == "fc-build-linux") | {name,status,busy,labels:[.labels[].name]}'
+done
+```
+
+Shared.Pos publish proof after the runner pod is online:
+
+```bash
+gh run list --repo astoltz/FlowerCore.Shared.Pos \
+  --workflow "Build, Test & Publish" --branch main --limit 5
+```
+
+If the latest run is still queued after runner registration, rerun the workflow
+from GitHub Actions and verify it lands on an `rke2-linux-*` runner.
+
+## Failure Notes
+
+- `actions/setup-dotnet` permission error at `/usr/share/dotnet`: check that
+  `DOTNET_INSTALL_DIR=/home/runner/.dotnet` and related cache env vars are
+  present on the runner pod.
+- `404` during runner registration: the fine-grained PAT is valid but missing
+  repository access for that repo. Add the repo to the PAT access list; the PAT
+  value does not change.
+- `Multi-Attach` volume error: only the Common runner uses a RWO PVC and it must
+  stay single-replica. New multi-replica runners use `emptyDir`.