Add ref as Alias for xref

Summary

Plan Runes uses {% ref "SPEC-008" /%} syntax for cross-references. The codebase has {% xref %}. Add ref as an alias for xref so both syntaxes work. This keeps the plan spec examples valid and provides a shorter, more natural syntax for inline references.

Acceptance Criteria

• {% ref "SPEC-008" /%} produces identical output to {% xref "SPEC-008" /%}
• ref registered as alias in the xref tag schema (packages/runes/src/tags/xref.ts)
• All xref attributes work with the ref alias (label, type)
• Test verifying alias produces same output

Approach

Add ref to the xref tag's aliases array in the Markdoc schema definition. No changes to the transform logic needed — Markdoc handles alias resolution automatically.