Support getting/setting git config values via but config

Add key/value handling to the Config subcommand so `but config <key> 
[value]` can get or set configuration values (e.g.,user.name / 
user.email). 

Implement config::handle to dispatch: setting writes to the
local repo config, getting uses existing lookup logic (including 
user.name/email helpers) and prints JSON when requested; fall back to 
the original show() behavior when no key is provided. Update CLI args to
accept optional key and value and wire the new handler into main.

This change was needed so that commands like `but config user.name 
<name>` actually set the name and `but config user.name` returns the 
effective value using the same lookup order as other config reads. It 
preserves existing behavior for showing all configuration and improves 
UX by supporting direct get/set operations.

Author: schacon

crates/but/src/args.rs

@@ -39,8 +39,13 @@ pub enum Subcommands {
         #[clap(long, short = 'b')]
         base: bool,
     },
-    /// Display configuration information about the GitButler repository.
-    Config,
+    /// Display or set configuration values for the repository.
+    Config {
+        /// Configuration key to get or set (e.g., user.name, user.email)
+        key: Option<String>,
+        /// Value to set (if provided, sets the key to this value)
+        value: Option<String>,
+    },
 
     /// Show operation history (last 20 entries).
     Oplog {

crates/but/src/config.rs

@@ -44,6 +44,42 @@ pub struct AiProviderInfo {
     pub model: Option<String>,
 }
 
+pub fn handle(current_dir: &Path, app_settings: &AppSettings, json: bool, key: Option<&str>, value: Option<&str>) -> Result<()> {
+    match (key, value) {
+        // Set configuration value
+        (Some(key), Some(value)) => {
+            set_config_value(current_dir, key, value)?;
+            if !json {
+                println!("Set {} = {}", key, value);
+            }
+            Ok(())
+        }
+        // Get specific configuration value
+        (Some(key), None) => {
+            let config_value = get_config_value(current_dir, key)?;
+            if json {
+                let result = serde_json::json!({
+                    "key": key,
+                    "value": config_value
+                });
+                println!("{}", serde_json::to_string_pretty(&result)?);
+            } else {
+                match config_value {
+                    Some(val) => println!("{}", val),
+                    None => println!("{} is not set", key),
+                }
+            }
+            Ok(())
+        }
+        // Show all configuration (existing behavior)
+        (None, None) => show(current_dir, app_settings, json),
+        // Invalid: value without key
+        (None, Some(_)) => {
+            Err(anyhow::anyhow!("Cannot set a value without specifying a key"))
+        }
+    }
+}
+
 pub fn show(current_dir: &Path, app_settings: &AppSettings, json: bool) -> Result<()> {
     let config_info = gather_config_info(current_dir, app_settings)?;
 
@@ -219,3 +255,36 @@ fn print_ai_provider(name: &str, provider: &AiProviderInfo) {
         println!("             {}", model_info.trim_start());
     }
 }
+
+fn set_config_value(current_dir: &Path, key: &str, value: &str) -> Result<()> {
+    let git_repo = git2::Repository::discover(current_dir)
+        .context("Failed to find Git repository")?;
+    let config = Config::from(&git_repo);
+    
+    config.set_local(key, value)
+        .with_context(|| format!("Failed to set {} = {}", key, value))
+}
+
+fn get_config_value(current_dir: &Path, key: &str) -> Result<Option<String>> {
+    let git_repo = git2::Repository::discover(current_dir)
+        .context("Failed to find Git repository")?;
+    let config = Config::from(&git_repo);
+    
+    // For getting values, use the same logic as the existing code
+    // which checks the full git config hierarchy (local, global, system)
+    match key {
+        "user.name" => config.user_name(),
+        "user.email" => config.user_email(),
+        _ => {
+            // For other keys, try to get the value from git config
+            let git_config = git_repo.config()?;
+            match git_config.get_string(key) {
+                Ok(value) => Ok(Some(value)),
+                Err(err) => match err.code() {
+                    git2::ErrorCode::NotFound => Ok(None),
+                    _ => Err(err.into()),
+                },
+            }
+        }
+    }
+}

crates/but/src/main.rs

@@ -110,8 +110,8 @@ async fn main() -> Result<()> {
             metrics_if_configured(app_settings, CommandName::Status, props(start, &result)).ok();
             Ok(())
         }
-        Subcommands::Config => {
-            let result = config::show(&args.current_dir, &app_settings, args.json);
+        Subcommands::Config { key, value } => {
+            let result = config::handle(&args.current_dir, &app_settings, args.json, key.as_deref(), value.as_deref());
             metrics_if_configured(app_settings, CommandName::Config, props(start, &result)).ok();
             result
         }