[MN4][P1] Complete the writeback and re-import workflow

[evan188199-tech]

Tracked by #10. Local plan: docs/roadmap/marginnote-4.md.

Goal (中文): 把 DeepTutor 的掌握度、错题和摘要写成 MN4 能稳定导入的 Markdown，并支持单笔记/目录 ZIP 下载。marginnote4:// 在真实验证前不启用。

Do you need to file a feature request?

• I have searched the existing feature requests and this feature request is not already filed.
• I believe this is a legitimate feature request, not just a question or bug.

Feature Request Description

Close the writeback loop so learning results created in DeepTutor can be imported back into MarginNote 4.

v1 already writes Markdown into deeptutor-notes/ with frontmatter for mastery_path_id, knowledge_point_id, and a reserved source_url. This is not yet a verified MN4 import workflow, and users have no download/export helper.

Scope:

• Generate Markdown that MN4 can stably import, preserving document, page, mastery, wrong-question analysis, and knowledge-point references
• Offer single-note download and a ZIP of the writeback directory
• Show generation results and recoverable errors in the MN4 detail Writeback view
• Keep marginnote4:// disabled until a real URL scheme is validated against MN4; use ordinary source references until then
• Default writeback directory remains deeptutor-notes/ under the export root unless the user configured another writable path

Related Module

Knowledge Base Management

Use Case

After a mastery review, DeepTutor writes a summary card. The learner downloads the note or the writeback ZIP, imports it into MN4, and sees mastery/wrong-question context next to the original PDF/EPUB notes.

Additional Context

Priority: P1
Phase: M2
Status: Planned
Depends on: real export-format validation; dedicated notebook detail
Blocked until verified: marginnote4:// deep links

Key files

• deeptutor/capabilities/marginnote/tools.py
• deeptutor/capabilities/marginnote/data/export_adapter.py
• web/components/knowledge/KnowledgeBaseDetail.tsx

Acceptance criteria

• Writeback Markdown preserves document, page, mastery, wrong-question analysis, and knowledge-point IDs.
• Users can download one note and a ZIP of the writeback folder.
• Generation success and recoverable failures are visible in UI.
• marginnote4:// is not advertised until validated against a real MN4 app; otherwise source_url stays a normal reference.
• Unwritable writeback path, empty analysis, and missing mastery data have explicit fallbacks.

Success metric
A generated summary can be imported into MN4 without losing mastery or source references.