Avoid duplicate migrated skills (#57853)

## Summary

- Track existing skill file contents and instruction bodies before
migrating Rules to Skills
- Skip writing migrated skills when an equivalent skill already exists
- Cover duplicate detection across differently named skills

## Tests

- cargo test -p prompt_store rules_to_skills_migration

Release Notes:

- Fixed duplicate Skills being created when migrating Rules.
b49e5f19b9Richard Feldman committed on 5/27/2026, 4:46:54 PM· committed by GitHubparent32f2593
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