Task 398: Add ID Attributes to Headings in markdownToHtml

Status: DONE

Summary

The markdownToHtml function in templates.ts renders headings without id attributes, making intra-page anchor links impossible. For example, #航法系不一致の物理的原因 in a link won't resolve because the <h3> tag has no id. Add slugified IDs to all rendered headings to enable anchor linking.

Rationale

• CLAUDE.md: "Reports should link to other episode reports... use section anchors"
• Cross-episode.md has links to anchors within episode pages that can't resolve
• Summary sections already use slugify() for their id attributes — extend to inline headings

Implementation

1. In markdownToHtml, add id attribute to all heading tags using the existing slugify function
2. Verify no duplicate IDs are generated
3. Test that anchor links resolve correctly