feat: allow generated files to have custom permissions set via YAML

[cbrgm]

This pull request adds support for specifying file permissions (such as 0755 for executables) in the FileStructure configuration, allowing generated files to have custom permissions set via YAML. It introduces a new FileMode type to handle octal permissions, updates the file creation logic to respect these permissions, and provides comprehensive tests for the new functionality.

File permissions support and propagation:

• Introduced a FileMode type for file permissions, supporting YAML unmarshaling from octal strings (e.g., "0755", "0644") or integers, and added a Permissions field to FileStructure to allow specifying file permissions per file. [1] [2]
• Updated file creation and copying logic in cmd/structuresmith/app.go to apply the specified permissions when writing files, defaulting to 0644 if none are provided. [1] [2] [3]
• Ensured that directory processing (processDirectory) propagates the Permissions field to all files within the directory.

YAML configuration and validation:

• Updated the example anvil.yml to demonstrate how to specify permissions for scripts, making them executable by default.
• Added and extended tests in config_test.go to validate correct parsing, stringification, and propagation of permissions in YAML, including handling invalid values. [1] [2]

Testing and correctness:

• Added extensive tests in app_test.go to verify that files are created with the correct permissions, that templated and copied files respect permissions, and that permission propagation and merging logic work as expected.