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

[SawyerHood]

Superseded by #875.

Thanks for this — the change is real and it's in. I reviewed it against a local checkout and found four issues — the biggest being that any text between two --- lines was treated as frontmatter, so a document opening with a thematic break had its first section hidden from the editor, so rather than round-trip here I folded the fix into #875 alongside the other three open contributor PRs (#865, #868, #869, #870).

Your commits are preserved on that branch with your original authorship, and the follow-up commit credits you as co-author. The full reasoning and the live QA evidence are in the #875 description.

Closing this one in favour of #875.