feat: add config system with --config argument and priority-based configuration

Co-authored-by: aider (openai/andrew/openrouter/anthropic/claude-sonnet-4) <aider@aider.chat>

src/args.rs

@@ -99,6 +99,10 @@ pub struct ItemArgs {
  */
 #[derive(Parser, Debug, Default, Clone)]
 pub struct OptionsArgs {
+    #[arg(long, env("KEEP_CONFIG"))]
+    #[arg(help("Specify the configuration file to use"))]
+    pub config: Option<PathBuf>,
+
     #[arg(long, env("KEEP_DIR"))]
     #[arg(help("Specify the directory to use for storage"))]
     pub dir: Option<PathBuf>,