ci: route build to self-hosted Mac mini runner

docs is a private repo and the org's GitHub-hosted Actions minute budget
is exhausted. Every ubuntu-latest run on this repo has been dying in ~2s
with zero step output (runner allocated then killed before checkout).

Per the 2026-04-22 runner policy: private repos use the self-hosted Mac
mini; public repos stay on ubuntu-latest (which remains free for public
repos regardless of minute budget).

Switches `runs-on: ubuntu-latest` → `runs-on: self-hosted`. No other
changes. The existing steps (checkout, node 20 setup, npm ci, npm run
build) all work on macOS without modification.

If the Mac mini has more specific labels assigned (e.g. macOS, ARM64),
tighten this to a matrix later — for now `self-hosted` matches any
registered runner, which should route correctly as long as there's only
one in the org.

.github/workflows/ci.yml

@@ -6,7 +6,12 @@ on:
     branches: [main]
 jobs:
   build:
-    runs-on: ubuntu-latest
+    # Self-hosted Mac mini — this repo is private and the org's
+    # GitHub-hosted minute budget is exhausted (every ubuntu-latest job
+    # dies in 2s with no step output). Per the 2026-04-22 carve-out:
+    # private repos run on self-hosted; public repos use ubuntu-latest
+    # (still free).
+    runs-on: self-hosted
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4