Add ChangeLog for 0.18.0

Documents the changes for the upcoming 0.18.0 release:

Breaking Changes

• Flake validation enabled by default - All pipelines now automatically get flake-show, flake-lock, and strict-inputs validation when gitlab.ci.enable = true

New Features

• child-pipelines-auto-discover option for automatic test flake discovery
• Improved canary test using real downstream project
• Better debugging with visible before_script in child pipeline jobs

🤖 Generated with Claude Code