Add placeholder (...) entries support

[plx]

Summary

• Implements support for placeholder entries (...) in navigation guides
• Allows indicating the presence of unlisted files without explicit enumeration
• Adds comprehensive validation and error handling for placeholder rules

Changes

• Added Placeholder variant to FilesystemItem enum
• Updated parser to recognize ... as placeholder syntax
• Implemented syntax validation:
  • Placeholders cannot have children
  • Placeholders cannot be adjacent to each other
• Implemented semantic validation:
  • Placeholders must refer to at least one unlisted item
  • Cannot be used in empty directories
• Added comprehensive unit tests for all placeholder functionality
• Updated README.md with usage examples and rules

Example Usage

<agentic-navigation-guide>
- src/
  - main.rs # Entry point
  - ... # Other source files
- docs/
  - README.md
  - api.md
  - ... # Additional documentation
</agentic-navigation-guide>

Test Plan

• All existing tests pass
• New unit tests for placeholder parsing
• New unit tests for placeholder syntax validation
• New unit tests for placeholder semantic validation
• Manual testing with valid and invalid placeholder usage
• cargo fmt and cargo clippy pass without warnings

🤖 Generated with Claude Code