Prevent duplicate files for existing Readwise books

[javenfang]

Summary

This PR prevents the Obsidian plugin from creating a new suffixed Markdown file when an export artifact contains a filename for a book that the vault already knows by book_id.

The plugin now resolves the write target through the existing booksIDsMap before writing highlight files. If the same book ID already has a known vault path, the sync updates that stable path instead of trusting a newly exported path such as Book Title (467).md.

User impact / bug report

This comes from a recurring real-world sync failure in a private Obsidian vault. The vault repeatedly accumulated numeric-suffix duplicates under Readwise/, for example paths shaped like Title (467).md and Title (485).md. In one cleanup pass, there were 624 duplicate Markdown files across three stable Readwise book/document IDs, with 208 suffixed copies per ID.

The pattern suggests the export artifact can sometimes contain a new suffixed filename for content that is already represented in the vault. Since the current plugin maps filePath -> bookID and writes to the artifact filename directly, those suffixed filenames become new files even when the same book_id already exists locally.

What changed

• Added stablePathForBookExport() to choose the existing path for the same book ID before creating a new file.
• Preserved user-renamed paths by preferring the existing mapped path.
• Kept full document content paths separate from normal highlight paths.
• Added regression tests for duplicate numeric suffix paths and existing duplicate candidates.

Validation

• npm test
• npm run typecheck
• npm run build