## CHANGES
- Create `PatternApplyRequest` struct for request body parsing
- Implement `ApplyPattern` method for POST /patterns/:name/apply
- Register manual routes for pattern operations in `NewPatternsHandler`
- Refactor `Get` method to return raw pattern content
- Merge query parameters with request body variables in `ApplyPattern`
- Use `StorageHandler` for pattern-related storage operations
- Add new template package to handle variable substitution with {{variable}} syntax
- Move substitution logic from patterns to centralized template system
- Update patterns.go to use template package for variable processing
- Support special {{input}} handling for pattern content
- Update chatter.go and rest API to pass input parameter
- Enable multiple passes to handle nested variables
- Report errors for missing required variables
This change sets up a foundation for future templating features like front matter
and plugin support while keeping the substitution logic centralized.
