fix: preserve arbitrary Markdown entry content

[rogerchappel]

Summary

• serialize frontmatter strings and tag arrays with JSON-compatible YAML values
• choose code fences longer than every captured backtick run and recover text by its indexed UTF-8 byte length
• render source and tags as escaped, delimiter-safe code spans in stored entries and exports
• document the updated format while retaining legacy fixed-fence reads

Commits

• 0e05e89 test: cover arbitrary Markdown entry content
• 94afe88 fix: serialize Markdown entries safely
• 96c23c0 docs: describe robust entry serialization

Verification

• npm ci
• npm test (15/15 tests pass)
• npm run release:check
• git diff --check origin/main...HEAD
• commit identity guard: all 3 commits are authored and committed by Roger Chappel miscanalysis@gmail.com