feat(ci): auto-pin publish workflows to freshly built runner image

build-image.yml now pushes a moving :latest tag alongside :<short-sha>,
and a follow-up pin job (running in :latest) rewrites the image pin in all
three publish-*.yml to the new sha and commits it back to master with
FAPAT. Publish workflows keep immutable sha pins, kept current with no
manual bump. No rebuild loop: the pin commit only touches workflow_call
files. Root README image-flow section updated to match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build-image.yml

@@ -46,10 +46,54 @@ jobs:
 
           # --insecure: push target is the plain-HTTP Gitea registry.
           # Base image pull (docker.io) stays HTTPS — no --insecure-pull.
+          # :latest is a convenience tag for internal CI (the pin job below
+          # runs in it). Publish workflows stay pinned to the immutable sha.
           /kaniko/executor \
             --context dir:///tmp/src \
             --dockerfile /tmp/src/Dockerfile \
             --destination "${IMAGE}:${SHORT_SHA}" \
+            --destination "${IMAGE}:latest" \
             --insecure
 
-          echo "Pushed ${IMAGE}:${SHORT_SHA}"
+          echo "Pushed ${IMAGE}:${SHORT_SHA} and ${IMAGE}:latest"
+
+  pin:
+    needs: build
+    runs-on: ubuntu-latest
+    container:
+      # Freshly built runner image: git + GNU sed baked in, and it has no
+      # non-shell ENTRYPOINT, so it works as a job container as-is (unlike the
+      # kaniko image above). Pulls :latest that the build job just pushed.
+      image: pi5-16.local:3005/mikkeli/novoyuuparosk-wiki-runner:latest
+    steps:
+      - name: Repoint publish workflows at the new image tag
+        env:
+          # Full-access PAT — needs contents:write to push the pin commit.
+          # github.token is read-only here; PKGRW_PAT is registry-only.
+          FAPAT: ${{ secrets.FAPAT }}
+          URL_TO_GITEA: ${{ vars.URL_TO_GITEA }}
+        run: |
+          set -eu
+          HOST="${URL_TO_GITEA#http://}"
+          SHORT_SHA=$(echo "${GITHUB_SHA}" | cut -c1-7)
+
+          git clone "http://mikkeli:${FAPAT}@${HOST}/mikkeli/novoyuuparosk-auto-wiki" repo
+          cd repo
+
+          for f in .gitea/workflows/publish-songs.yml \
+                   .gitea/workflows/publish-ses.yml \
+                   .gitea/workflows/publish-tech.yml; do
+            sed -i -E "s#(novoyuuparosk-wiki-runner:)[A-Za-z0-9._-]+#\1${SHORT_SHA}#" "$f"
+          done
+
+          if git diff --quiet; then
+            echo "Pins already at ${SHORT_SHA}; nothing to commit."
+            exit 0
+          fi
+
+          git config user.email "actions@novoyuuparosk.org"
+          git config user.name "wiki-runner image bot"
+          git commit -am "build: pin runner image to ${SHORT_SHA}"
+          # Pin commit only touches publish-*.yml (on: workflow_call) — triggers
+          # nothing, so no rebuild loop. Fails loudly if master moved meanwhile.
+          git push origin HEAD:master