ci(bluebuild): pin blue-build/github-action to commit SHA

Replace @v1 with @24d146df25adc2cf579e918efe2d9bff6adea408 (the commit
v1 currently resolves to). Tag pins on third-party actions are mutable
— a maintainer or attacker can re-point v1 at a malicious commit and
silently change what runs on every push.

Trailing comment '# v1' preserves human readability for future bumps.

Refs: 9-agent CI hardening wave (agent 8), 2026-05-05.

.github/workflows/build-bluebuild.yml

@@ -57,12 +57,12 @@ jobs:
           df -h
 
       # BlueBuild action wraps: image build, cosign sign (keyless via
-      # Sigstore), GHCR push. To pin to a commit SHA in a follow-up
-      # once the workflow shape stabilises (CI hardening agent 8,
-      # 2026-05-05 wave).
+      # Sigstore), GHCR push. Pinned to a commit SHA per CI hardening
+      # agent 8 (2026-05-05 wave). The trailing comment records the
+      # tag the SHA resolved from, so future bumps stay legible.
       - name: Build + push veilor-os OCI
         id: bluebuild
-        uses: blue-build/github-action@v1
+        uses: blue-build/github-action@24d146df25adc2cf579e918efe2d9bff6adea408 # v1
         with:
           recipe: bluebuild/recipe.yml
           registry_token: ${{ secrets.GITHUB_TOKEN }}