📝 Document CI runner-image bump ritual

build:runner-image broke the script prefix convention: build is a bare
single-tool command, and no tier fits a docker-daemon + registry-cred action,
so the script leaves package.json and is invoked directly. CONTRIBUTING gains
a 'CI runner image' section recording where the image pushes
(gitea.e1nsnull.de/tmu/act-ci:<version>), the .node-version coupling, and the
three-step Node-bump ritual.

package.json

@@ -38,7 +38,6 @@
     },
     "scripts": {
         "build": "tsc -p tsconfig.build.json",
-        "build:runner-image": "./scripts/runner-image.sh",
         "prebuild": "rm -rf dist",
         "check": "npm run check:tsc && npm run check:oxlint && npm run check:oxfmt && npm run check:cspell",
         "check:cspell": "cspell lint ${LEFTHOOK_FILES:-.}",