fix: make mode export/import paths slug-independent to support renaming

[ghost]

Summary

This PR fixes issue #6229 where rules files were written to the wrong directory when users changed the slug in exported YAML files before importing.

Problem

When exporting a mode, the relativePath for rule files included the rules-<slug> prefix (e.g., rules-old-slug/rule.xml). If a user changed the slug in the exported YAML file before importing, the rules were written to a directory based on the old slug instead of the new one.

Solution

1. Export function: Changed to calculate relative paths from the mode's rules directory, making them slug-independent (e.g., rule.md instead of rules-slug/rule.md)
2. Import function: Added backwards compatibility to strip old format prefixes when found, ensuring existing exports continue to work

Changes

• Modified exportModeWithRules in CustomModesManager.ts to use path.relative(modeRulesDir, filePath)
• Modified importRulesFiles in CustomModesManager.ts to detect and strip rules-<slug>/ prefixes for backwards compatibility
• Added comprehensive tests for slug renaming scenarios
• Updated existing export tests to verify the new path format

Testing

• All existing tests pass ✅
• Added new tests specifically for slug renaming scenarios
• Added tests for backwards compatibility with old export format
• Added tests for new export format

Impact

Users can now:

• Rename modes by editing the slug in exported YAML files
• Share modes with different names
• Create variations of existing modes with new slugs

This change is backwards compatible - existing exports will continue to work correctly.

Fixes#6229

---

Important

Fixes slug-dependent paths in mode export/import, allowing slug renaming and ensuring backward compatibility.

• Behavior:
  • exportModeWithRules in CustomModesManager.ts now calculates relative paths from the mode's rules directory, making them slug-independent.
  • importRulesFiles in CustomModesManager.ts strips rules-<slug>/ prefixes for backward compatibility.
• Testing:
  • Added tests for slug renaming scenarios and backward compatibility in CustomModesManager.spec.ts.
  • Updated existing export tests to verify new path format.
• Impact:
  • Users can rename modes by editing the slug in exported YAML files.
  • Backward compatibility ensures existing exports continue to work.

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[daniel-lxs]

Closed by #6186