chore(ci): adopt .runtime-version push-mode cascade signal

Background: post-2026-05-06 SCM is Gitea, not GitHub. Gitea 1.22.6 has
no repository_dispatch / workflow_dispatch trigger API (empirically
verified across 6 candidate paths in molecule-core#20 issuecomment-913).
The molecule-core/publish-runtime.yml cascade therefore cannot fire
templates via curl-dispatch — pivots to push-mode instead.

This PR is the consumer side of that pivot:

- .runtime-version file at repo root — single line, plain version
  string. Currently 0.1.129 (latest published as of 2026-05-07).
  publish-runtime overwrites this on each cascade.

- publish-image.yml gains a resolve-version job that reads the file
  and forwards the value to the reusable build workflow as the
  third-priority source in the resolution chain.

Sequencing context: this PR (and 8 sibling PRs to the other template
repos) MUST land before molecule-core#20 v2 is merged.

Refs molecule-core#14, molecule-core#20.

.github/workflows/publish-image.yml

@@ -25,7 +25,33 @@ permissions:
   packages: write
 
 jobs:
+  # The `.runtime-version` file is the push-mode cascade signal post-
+  # 2026-05-06: when molecule-core/publish-runtime.yml ships a new
+  # version to PyPI, it does NOT call repository_dispatch (Gitea 1.22.6
+  # has no such endpoint — empirically verified molecule-core#20).
+  # Instead it git-pushes an updated `.runtime-version` to each template,
+  # which trips this workflow's `on: push: branches: [main]` trigger.
+  # This job reads that file and forwards the version to the reusable
+  # build workflow.
+  resolve-version:
+    runs-on: ubuntu-latest
+    timeout-minutes: 2
+    outputs:
+      version: ${{ steps.read.outputs.version }}
+    steps:
+      - uses: actions/checkout@v4
+      - id: read
+        run: |
+          if [ -f .runtime-version ]; then
+            v="$(head -n1 .runtime-version | tr -d '[:space:]')"
+            echo "version=$v" >> "$GITHUB_OUTPUT"
+            echo "resolved runtime version: $v"
+          else
+            echo "no .runtime-version file present — falling through to Dockerfile default"
+          fi
+
   publish:
+    needs: resolve-version
     uses: Molecule-AI/molecule-ci/.github/workflows/publish-template-image.yml@main
     secrets: inherit
     with:
@@ -33,4 +59,4 @@ jobs:
       # version PyPI just published. Forwarded as a docker --build-arg
       # so the cache key changes per-version and pip install resolves
       # freshly. Empty on push/PR — falls back to requirements.txt pin.
-      runtime_version: ${{ github.event.client_payload.runtime_version || inputs.runtime_version || '' }}
+      runtime_version: ${{ github.event.client_payload.runtime_version || inputs.runtime_version || needs.resolve-version.outputs.version || '' }}

.runtime-version

@@ -0,0 +1 @@
+0.1.129