All other .gsd/ state files use uppercase naming (DECISIONS.md,
REQUIREMENTS.md, PROJECT.md, etc). This renames the canonical
preferences file to PREFERENCES.md while keeping a migration
fallback — the loader checks PREFERENCES.md first, then falls
back to lowercase preferences.md for existing installations.
Closes#2700
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
