feat: finish incremental design and update docs

[anaselmhamdi]

Complete incremental sync implementation for Notion source and comprehensive documentation for humans and AI agents to implement incremental sync in other connectors.

Type of Change

• New feature

Changes Made

• Add get_records_after() to Notion source for incremental sync
  • get_pages_after(): Search API with client-side filtering
  • get_blocks_markdown_after(): Database query with combined timestamp + user filters
  • Respects database_ids and database_filters configuration
• Add incremental support to file and logger destinations
  • file: append mode with temp file handling in finalize()
  • logger: logs sync mode completion
• Update documentation:
  • README: expanded incremental sync section with flow diagram, config options, examples
  • CLAUDE.md: implementation guidance with code examples for AI agents
  • docs/contributing/adding-sources.md: get_records_after() implementation guide
  • docs/contributing/adding-destinations.md: finalize() patterns for sync modes
  • docs/ai-connector-guide.md: incremental sync templates and decision trees
• Add example configs for incremental sync:
  • Destinations: BigQuery, file, logger
  • Sources: Notion, HubSpot, GSheets
• Add live integration tests for BigQuery incremental sync

Testing

• Unit tests added/updated
• Manual testing performed
• Existing tests pass (uv run pytest)

Manual testing:

• Ran Notion incremental sync with live credentials
• First run: full refresh behavior (fetched all data)
• Second run: correctly found only 1 page edited after last_run
• Verified blocks fetched and appended to destination file

Checklist

• Code follows project style (make format)
• Self-review completed
• Documentation updated (if needed)