ci: self-hosted warden-sdk runner registered + docs/ci-cd.md (P5)

Registered a 3rd repo-scoped runner on bfe-mpc-0640 (label warden-sdk,
bfe-mpc-0640-warden-sdk) per ADR-0004. It is offline pending the sudo-gated steps
(service install + CPUQuota/MemoryMax cgroup cap + toolchain/python provisioning),
documented in docs/ci-cd.md as a [maintainer] handoff. kernel-build stays dispatch-gated
and passes JOBS=4 as a belt-and-braces resource bound.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017wB8KB3MMQztRDXCMCkPrf

.github/workflows/ci.yml

@@ -115,6 +115,7 @@ jobs:
       - name: build zImage + rv1106-warden.dtb
         env:
           WORK: ${{ github.workspace }}/kbuild-out
+          JOBS: 4   # belt-and-braces bound in addition to the runner's cgroup cap
         run: bash build/build-kernel.sh
       - uses: actions/upload-artifact@v4
         with:

docs/ci-cd.md

@@ -0,0 +1,45 @@
+# CI/CD
+
+`.github/workflows/ci.yml` — everything portable runs on GitHub-hosted
+`ubuntu-latest`; only the heavy kernel build uses the self-hosted runner.
+
+## Jobs
+
+| Job | Runner | What it does |
+|---|---|---|
+| `test` | ubuntu-latest | `cargo test` (sim + config-lint) + `cargo-llvm-cov` line coverage on `sim`; outputs `passed`/`coverage`. |
+| `mcdc` | ubuntu-latest | 100% MC/DC enforced on every `drivers/*/test` (gcc-14 `-fcondition-coverage`). |
+| `bench` | ubuntu-latest | Smoke-runs the sim micro-benchmarks; emits ns/op trend JSON. |
+| `patches-apply` | ubuntu-latest | Fetches pristine linux-6.18.46 (cached, sha256-verified) and applies `patches/*` in order. |
+| `kernel-build` | **[self-hosted, warden-sdk]** | `build/build-kernel.sh` → `zImage` + `rv1106-warden.dtb`, uploaded as an artifact. Dispatch-gated until the runner is fully provisioned (below). |
+| `badges` | ubuntu-latest | Renders loc/tests/coverage shields on push to `bringup` (`[skip ci]` + `paths-ignore` loop guard). |
+
+## The self-hosted runner (`bfe-mpc-0640`)
+
+A **third** repo-scoped runner instance on `bfe-mpc-0640` (alongside `flare` and
+`flare-edge`), registered with the label **`warden-sdk`** as
+`bfe-mpc-0640-warden-sdk`, in `~/actions-runner-warden-sdk`. The registration is
+done; the following steps need **`user`'s sudo on 0640** and are not automatable
+from the dev box:
+
+1. **Install as a service** (persistence): `cd ~/actions-runner-warden-sdk &&
+   sudo ./svc.sh install user && sudo ./svc.sh start`. Until then the runner is
+   *offline* and `kernel-build` only runs when dispatched against an online runner.
+2. **Resource cap** (protect the shared host): a drop-in at
+   `/etc/systemd/system/actions.runner.bfe-noah-warden-sdk.*.service.d/*.conf` with
+   `CPUQuota=400%` + `MemoryMax=6G`, then `sudo systemctl daemon-reload`. The build
+   inherits that cgroup. (The workflow also passes `JOBS=4` as a belt-and-braces bound.)
+3. **Kernel cross toolchain**: `build-kernel.sh` needs the arm cross compiler on
+   PATH — set `SDK_TC` to the dir holding `arm-rockchip830-linux-uclibcgnueabihf-*`
+   (the Luckfox SDK toolchain, as flare-edge's runner has), or install
+   `gcc-arm-linux-gnueabihf` and pass `CROSS_COMPILE=arm-linux-gnueabihf-` (the
+   kernel is freestanding, so a generic arm cross compiler links it).
+4. **`python`** (not python3): the kernel build calls bare `python`; provide a
+   project-local venv or a `python`→`python3` shim on the runner's PATH.
+
+Host build deps (sudo): `dtc bc flex bison libssl-dev` (already present on 0640).
+
+## Badges
+
+Static shields SVGs are committed by the `badges` job (private repo can't use
+dynamic shields). The GitHub-native `ci.yml` status badge works live regardless.