docs(snapshot[README]): add comprehensive doctest-based README for snapshot module

The snapshot module has been enhanced with a detailed README.md that serves
both as documentation and executable tests. This commit introduces a thorough
guide for users to understand and leverage the snapshot functionality.

- **Value Proposition Section**: Added clear explanation of benefits the
  snapshot module provides for tmux automation and management

- **Progressive Learning Structure**: Organized examples from basic to advanced,
  building user knowledge incrementally

- **Executable Documentation**: Converted all examples to doctests that run
  with pytest, verified with 19 passing tests

- **Environment-Resilient Tests**: Implemented fallback patterns to ensure
  tests pass in varied environments (CI, local development)

- **Real-World Use Cases**: Added practical examples showing how to apply
  snapshots for:
  * Testing tmux automations with before/after comparison
  * Session state backup and restoration
  * Configuration comparison between windows/sessions
  * Content monitoring and change detection
  * Saving and restoring window arrangements

- **Best Practices Section**: Included guidance on effective snapshot usage,
  addressing memory concerns, and optimization tips

- **API Overview**: Added summary of snapshot module structure and
  available methods

- **User-Friendly Format**: Included import statements in each section for
  copy-paste friendliness and added "Tip" sections for additional context

The README now serves multiple purposes: code verification via tests,
educational resource for new users, and comprehensive reference for
experienced developers. All doctests are confirmed working with

Author: tony