Feature Proposal: Configuration File Support

[psschwei]

Developers may want to set their own default backend, model, and credentials once and have them work across all their programs, but currently must hardcode these settings in every script. There's got to be a better way! 😄

One thing we could do is add configuration file support with clear precedence:

• User config: ~/.config/mellea/config.toml (global defaults)
• Project config: ./mellea.toml (project overrides)
• Explicit parameters: runtime overrides

Some possible benefits include:

• Configure once, use everywhere
• Can keep credentials in user config or environment variables (so some security benefit)
• Share project configs for uniform settings (prod config, dev config, etc.)
• Change models by editing config, not code
• Shouldn't impact existing code, no breaking changes