Prototype: optional YAML front matter on generated markdown (#57)

[philips-clanker]

Summary

Prototype for #57 ("Adding YAML"): adds an opt-in setting, "Add YAML front matter to Markdown files" (off by default), that prefixes generated notes with a small, fixed set of front matter properties:

---
tags:
  - supernote
source: "[[Meeting notes.note]]"
device: "A5X2"
pages: 3
keywords:
  - "budget"
  - "Q3 plan"
---

• tags: [supernote] — lets a vault filter/query every Supernote-imported note (e.g. tag:#supernote or a Dataview FROM #supernote).
• source — a link back to the original .note/.spd file as a property (not just the existing body-text link), so it's usable in Obsidian's property search ([source]) and in Dataview/Bases link queries.
• device — the capturing device model (SupernoteX.header.APPLY_EQUIPMENT), omitted when the device didn't record one.
• pages — page count, useful for sorting/filtering long vs. short notes.
• keywords — the device's own "starred" keywords, deduplicated across all pages, omitted when there are none.

Serialization is hand-rolled (src/frontmatter.ts, no new dependency) using JSON.stringify for double-quoted YAML scalars, which safely escapes colons/quotes in note titles or keywords without needing a YAML library.

Kept intentionally small for a first pass — no template/Templater integration, no custom per-vault property mapping, no created/modified dates (the .note format doesn't carry a real creation timestamp; TFile.stat.ctime would just be vault-import time, which felt more misleading than useful to include by default). Posted a comment on #57 with the fuller brainstorm of what else front matter could carry, for discussion before committing to a fixed schema.

Test plan

• npx vitest run — all existing tests plus new src/frontmatter.test.ts pass
• npx tsc --noEmit — clean
• npm run lint — clean (no new warnings)
• npm run build — builds cleanly
• Manual check in an actual Obsidian vault (not done in this environment)