feat(bookstack): add configurable export options

[hexsm]

Summary

Adds configurable BookStack export options for selecting the source scope, output type, file format, and path structure.

This supersedes #80 and #82 and includes the combined BookStack export changes in one PR.

Addresses #75.

What Changed

• Adds BookStack source options:
  • scope=pages|books|shelves
  • output=pages|books
  • format=txt|md|html|pdf
  • structure=flat|hierarchical
• Uses BookStack native export endpoints for pages and books.
• Supports syncing:
  • individual pages
  • pages from books
  • whole book exports
  • pages or books from shelves
• Adds hierarchical paths for shelf/book/chapter structure where available.
• Updates BookStack checksums so whole-book exports change when nested page content changes.
• Adds BookStack parser and connector tests.
• Documents the new BookStack options in the guide.

Examples

# Sync all pages from all BookStack books as Markdown files.
oikb sync bookstack: --kb-id your-kb-id

# Sync one specific BookStack page.
oikb sync 'bookstack:12?scope=pages' --kb-id your-kb-id

# Sync one whole BookStack book as a PDF file.
oikb sync 'bookstack:12?scope=books&output=books&format=pdf' --kb-id your-kb-id

# Sync all pages from one shelf and preserve shelf/book/chapter paths.
oikb sync 'bookstack:5?scope=shelves&structure=hierarchical' --kb-id your-kb-id

Validation

• pytest: 28 passed
• compileall -f src/oikb tests: passed
• git diff --check: passed
• Live BookStack diff checks:
  • page scope
  • default book pages
  • whole-book markdown/html/pdf exports
  • plaintext exports
  • hierarchical shelf pages
  • hierarchical shelf book exports
• Live upload/idempotency checks:
  • single page sync, then unchanged repeat
  • whole-book sync, then unchanged repeat
  • hierarchical shelf sync, then unchanged repeat