Add image cache lookup scripts and templates

Summary

• Add generic scripts for finding pre-built Docker images without rebuilding
• Add CI templates for source change detection and upstream image lookup
• Enable downstream repos to find upstream images without maintaining submodules

New Scripts (scripts/bash/)

Script	Purpose
find-last-source-commit.sh	Find the most recent commit that changed source file patterns
get-cached-image.sh	Check if a Docker image exists in a registry for a commit
find-upstream-image.sh	Find images from upstream repos (combines git fetch + lookup)

New Template (templates/source_change_detection.gitlab-ci.yml)

• .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

Use Cases

1. Building repo (hive): Find last source commit, check if image exists, skip rebuild
2. Downstream repo (clive): Fetch upstream repo, find last source commit, use pre-built image

Example Usage

# Downstream repo finding upstream image
find-upstream-image.sh \
  --repo-url=https://gitlab.syncad.com/hive/hive.git \
  --registry=registry.gitlab.syncad.com/hive/hive \
  --patterns="libraries/,programs/,CMakeLists.txt,Dockerfile"

Test plan

• Test scripts locally with hive repo
• Test cross-repo lookup (e.g., clive finding hive image)
• Validate CI template syntax