feat: add overwrite option to protect files from being overwritten

[cbrgm]

Summary

• Adds overwrite field to FileStructure that controls whether existing files should be overwritten during render
• When set to false, files that already exist on disk will be skipped, preserving user customizations
• Useful for files like .env configurations that users customize after initial generation

Changes

• Add Overwrite *bool field to FileStructure (defaults to true if nil for backwards compatibility)
• Add StatusSkipped and cyan "skip:" output in diff
• Add applySkipLogic() to classify files based on existence and overwrite setting
• Propagate overwrite setting through directory processing
• Add comprehensive tests for the new functionality
• Update example config with overwrite: false demonstration

Usage

templateGroups:
  config:
    - destination: "config/.env"
      content: |
        API_KEY=changeme
        DATABASE_URL=changeme
      permissions: "0600"
      overwrite: false  # Protect user customizations

Behavior

overwrite	File exists	Result
nil (default)	Yes	Overwrite
true	Yes	Overwrite
false	Yes	Skip (preserve existing)
false	No	Create (file doesn't exist yet)

Closes #34