Add configuration file support with --init flag

[Boomatang]

Closes #16

Summary

Adds support for a ZON-based configuration file so users can set default behavior (clone path, clone mode, shallow clones, log level) without repeating CLI flags or relying on environment variables.

A new --init flag generates a default configuration file at the XDG-compliant location. On startup, the configuration file is loaded and its values are applied, with CLI flags and environment variables taking precedence.

Precedence

Settings are resolved in the following order (most specific wins):

CLI flags > Environment variables (GRAB_PATH) > Config file > Built-in defaults

Acceptance Criteria

• File format: Configuration uses ZON (Zig Object Notation), parsed natively with std.zon.parse -- no external dependencies needed.
• File location: Follows XDG Base Directory specification -- checks $XDG_CONFIG_HOME/grab/config.zon first, falls back to $HOME/.config/grab/config.zon.
• Settings: All four settings from the spec are supported: path, action, shallow, and log_level, with the specified types and defaults.
• Example config: The generated default config file includes all settings with comments explaining each option, and path is commented out so it defaults to null.
• Precedence: CLI flags override environment variables, which override config file values, which override built-in defaults.
• Creation: grab --init generates a config file with defaults. If the file already exists, it warns and exits without overwriting.
• Error handling: Invalid ZON in the config file produces a parse error and halts execution -- no silent fallback to defaults.
• No config file: The config file is entirely optional. Without one, the tool behaves exactly as before.