Fix Docs YAML frontmatter parsing

[amadad]

Summary

• parse fenced YAML frontmatter through one shared Docs boundary
• use a string frontmatter title (or the document H1) for navigation and mentions
• keep frontmatter out of the rich editor and previews while preserving it byte-for-byte on save
• keep frontmatter-managed filenames stable instead of auto-renaming them

Problem

Docs previously derived a note title from the first raw Markdown line and passed the whole file to Tiptap. Wiki pages therefore appeared as repeated --- entries, rendered metadata as document content, and risked rewriting frontmatter after an edit.

Verification

• pnpm exec turbo run test --filter=bb-plugin-simple-notes --force — 46 passed
• pnpm exec turbo run typecheck --filter=bb-plugin-simple-notes --force — passed
• pnpm exec turbo run build --filter=bb-app --force — passed
• pnpm exec turbo run smoke:tarball --filter=bb-app --force — passed
• exercised against a 433-document Markdown vault — zero delimiter-derived titles