feat(bookstack): support syncing pages from a specific book

[hexsm]

Summary

Adds optional BookStack page filtering by numeric book ID.

BookStack sources now support:

oikb sync bookstack: --kb-id <kb>     # all visible pages, unchanged behavior
oikb sync bookstack:12 --kb-id <kb>   # pages from BookStack book ID 12

Why

The BookStack connector previously synced all visible pages from the instance. This change allows syncing a specific BookStack book without relying on slugs, which can change over time.

Changes

• Parse an optional numeric book ID from bookstack: sources.
• Pass book_id into BookStackConnector.
• Apply BookStack's filter[book_id] parameter when listing pages.
• Keep bookstack: behavior unchanged for syncing all visible pages.
• Reject invalid BookStack source values like bookstack:abc.
• Document BookStack usage in README.md and docs/guide.md.
• Add BookStack parser and request tests.

Validation

• python -m pytest
  • 7 passed
• python -m compileall -f src/oikb/connectors/bookstack.py src/oikb/cli.py tests/test_bookstack.py
• Live BookStack check using local credentials:
• bookstack: listed all visible pages.
• bookstack:1 listed only pages from book ID 1.
• Verified the BookStack API filter returns only pages matching the selected book_id.

[hexsm]

Closing this in favor of #84.