fix: separate duplicate workspace root groups

[vycdev2]

Summary

• Keep Todo files from workspace roots with the same display name in separate files-view groups.
• Disambiguate colliding labels with workspace paths while leaving repeated files from one root unchanged.
• Add focused regression coverage and document the fix in the unreleased changelog.

Verification

• npm install - passed; npm reported 30 audit vulnerabilities and deprecation warnings.
• Initial RED on untouched upstream/develop: the focused grouping test failed because the intended helper did not exist.
• Parent-review RED: repeated files from one workspace root were incorrectly given path-qualified labels.
• Focused GREEN: npm test -- --grep 'Todo file root grouping' - passed, 2/2.
• npm test - passed, 136/136.
• npm run compile - passed.
• npm run format:check - passed.
• git diff --check - passed.

Risk

Low. Only colliding display names from distinct workspace paths are disambiguated; unique roots and repeated files under one root retain their existing labels.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.