feat(export): pure frontmatter + slug + markdown + jsonl + manifest composers
Pure compose layer for F5 (Export). slugifyTitle, composeFilename,
composeFrontmatter, composeMarkdown, composeIndexJsonl, composeManifest
+ ExportNote/ExportNoteMedia/ExportNoteTag types. No fs deps.
24 unit tests covering normal cases + edge cases (null title,
forbidden chars, multiline summary needing block scalar, colon
needing single-quote, image numbering by id8__n.ext).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
8e09464d5e