v1.3.1: safer README handling

• Damaged markers leave your README untouched (with a warning) instead of adding a second block.
• readme.md / Readme.md are found and committed correctly on Linux runners.
• The generated block now says it's regenerated; use update-readme: false to lay images out yourself.
• The installer won't overwrite a profile-effects.yml workflow that isn't from CustomizeYouProfile (use FORCE=1 to replace it).