start contextuel : am start sans argument lance l'agent par defaut du projet (issue #30) - resolution pure resolve_start_target, restart identique, REPL sans agent obligatoire

src/cli.rs

@@ -298,8 +298,8 @@ pub enum Command {
 /// Arguments shared by start and restart.
 #[derive(Args, Debug, Clone, Default)]
 pub struct StartArgs {
-    /// Agent name, alias, or group:<name>
-    pub agent: String,
+    /// Agent name, alias, or group:<name> (defaults to the project's default agent)
+    pub agent: Option<String>,
     /// Run detached in the background; output goes to the agent log file
     #[arg(short = 'b', long, conflicts_with = "foreground", action = ArgAction::SetTrue)]
     pub background: bool,