ci: fix speedrun sync to clean up stale files

[tu11aa]

Summary

• Add --delete flag to rsync in the speedrun sync workflow so files removed from scaffold-stark-2 are also removed from speedrunstark — this was the root cause of growing drift and manual maintenance
• Move exclude/protect rules to a versioned filter file (.github/sync-speedrun-filter-rules) instead of inline --exclude flags — adding new speedrun-specific files now requires just one line change
• Add empty-commit guard so the workflow doesn't report failure when a merged PR only touches files outside the sync scope (e.g., README, CONTRIBUTING, tests)

Context

The basecamp sync workflow already uses rsync --delete but speedrunstark did not, causing files deleted from scaffold-stark-2 to persist indefinitely in speedrunstark. This required periodic manual cleanup.

Filter file structure

P <path>   → Protect (never delete from speedrunstark)
- <path>   → Exclude (never copy from scaffold-stark-2)

Protected files include speedrun-specific UI components, branding assets, fonts, .gitattributes merge strategies, and the challenge sync workflow.

Test plan

• Verify filter file covers all speedrun-specific files listed in base-challenge-template branch
• Trigger a test sync by merging this PR and confirming the workflow succeeds
• Verify no speedrun-specific files are deleted from base-challenge-template
• Verify stale files (if any) are cleaned up by --delete