Add custom Kingfisher container built from sporked feature branches

- Dockerfile: deterministic build from pinned CONTAINER_APP_VERSION + FEATURES
- Merges named feature branches at specific SHAs for reproducibility
- Switch CronJob to custom image with --clone-url-base and --all-organizations
- Add kingfisher to service-versions.yaml (version tracks upstream main SHA)
- Document spork container builds in new how-to card
- Document spork workflow in CLAUDE.md
- Update kingfisher service docs for custom image

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

containers/kingfisher/Dockerfile

@@ -41,7 +41,7 @@ RUN set -e; \
     done; \
     echo "Build tree ready at $(git rev-parse --short HEAD)"
 
-RUN cargo build --release -j 4 \
+RUN cargo build --release \
     && install -m 0755 target/release/kingfisher /usr/local/bin/kingfisher
 
 # Quick smoke-test