feat(config): support config.d/ directory for split configuration

[ferdinandyb]

Allow users to organize configuration into multiple files within a config.d/ directory. Files are loaded alphabetically and merged after the main config file, making it easier to manage modular config setups (e.g., separate files for permissions, plugins, theming).

Supports both ~/.config/opencode/config.d/ (global) and .opencode/config.d/ (project-specific) locations.

How did you verify your code works?

added testcases

Closes #9062