feature: /dld-reindex to sync references after refactors

[jimutt]

When code is refactored (files renamed, functions moved), the references field in decision records becomes stale. The @decision annotations in code survive refactors naturally since they move with the code, but the reverse index (decision → code) drifts.

Add a /dld-reindex skill (or similar) that:

• Scans all @decision(DL-XXX) annotations in the codebase
• Rebuilds the references field in each decision record to match current file paths and symbols
• Reports what changed

This treats references as derived data — similar to how INDEX.md is regenerated from decision files.