Add image cache lookup scripts and templates

Add generic scripts for finding pre-built Docker images:
- find-last-source-commit.sh: Find most recent commit that changed source files
- get-cached-image.sh: Check if Docker image exists for a commit
- find-upstream-image.sh: Find images from upstream repos (git fetch + lookup)

Add CI template (source_change_detection.gitlab-ci.yml) with:
- .detect_source_changes: Detect docs-only/tests-only changes
- .find_upstream_image: Look up images from upstream repos
- .check_local_image: Check local registry for cached images
- Skip rule templates for conditional job execution

This generalizes hive's get_image4submodule.sh pattern for use by any repo,
and enables downstream repos (clive, wax, etc.) to find upstream images
without maintaining submodules.