feat(skill-maker): add XML structure guidance and update examples

[schultzp2020]

Summary

Agents parse XML tags more reliably than markdown headings for skills with multiple semantic sections. This updates skill-maker to recommend and demonstrate XML structure throughout.

Changes

New files:

• references/xml-structure-guide.md — suggested tag patterns, when to use XML vs markdown, anti-patterns
• templates/router-skill-xml.md — XML-based starter template for router skills

SKILL.md updates:

• Uses <intake>, <routing>, <reference_index> tags (practices what it preaches)
• Adds XML structure guidance section with when-to-use decision criteria
• Updates audit and review checklists to consider XML for multi-section skills

Reference file updates:

• architecture-patterns.md — router structure section lists XML tags, router table example uses <intake>/<routing>
• anti-patterns.md — missing intake question fix shows <intake> example
• spec-guide.md — body section shows XML structure example

Audit fixes:

• xml-structure-guide.md examples use generic placeholder paths instead of paths from sibling skills
• SKILL.md routing table uses specific section names instead of vague "follow below"

Design decisions

• XML is presented as suggested patterns, not a rigid vocabulary — skills should invent tags that fit their domain
• Skill-maker phases (1–5) stay as markdown headings — sequential instructional content where order matters more than section lookup
• XML recommended for skills with semantically distinct sections (intake, routing, principles) that agents need to jump between