Fix sync workflow: exclude README.md from diff comparison

[ilonatommy]

The sync-runtime-skills workflow always detects changes because runtime-skills/README.md (containing sync metadata like "Last synced" timestamp) exists locally but not in upstream's .github/skills/ directory.

Fix: Change the diff command in the "Check for changes" step from:

diff -rq runtime-skills /tmp/runtime/.github/skills

to:

diff -rq --exclude='README.md' runtime-skills /tmp/runtime/.github/skills

This ensures the workflow only creates a PR when actual skill content has changed.