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

@@ -26,7 +26,7 @@ ARG FEATURES="feature/upstream/clone-url-base=677c7a5"
 # Limit parallelism to avoid overwhelming shared infrastructure.
 # cargo -j controls Rust compilation; CMAKE_BUILD_PARALLEL_LEVEL
 # controls the vectorscan/Boost cmake build (called from build.rs).
-ENV CMAKE_BUILD_PARALLEL_LEVEL=2
+ENV CMAKE_BUILD_PARALLEL_LEVEL=1
 
 RUN git clone https://forge.ops.eblu.me/eblume/kingfisher.git . \
     && git checkout "${CONTAINER_APP_VERSION}" \
@@ -46,7 +46,7 @@ RUN set -e; \
     done; \
     echo "Build tree ready at $(git rev-parse --short HEAD)"
 
-RUN cargo build --release \
+RUN cargo build --release -j 1 \
     && install -m 0755 target/release/kingfisher /usr/local/bin/kingfisher
 
 # Quick smoke-test