feat: detect shell for --kernel

- Move detect_shell() to new src/utils.rs for reuse
- Use detected shell in QEMU VM mode (bash when available)
- Update chroot.rs and qemu_vm.rs to use shared function

src/qemu_vm.rs

@@ -54,6 +54,9 @@ pub fn launch_qemu(config: QemuConfig) -> Result<()> {
         qemu_bin, get_arch_package_suffix(&config.arch), get_arch_package_suffix(&config.arch), get_arch_package_suffix(&config.arch)
     ))?;
 
+    // Detect the best available shell in the rootfs
+    let shell = crate::utils::detect_shell(&config.rootfs_path);
+    
     // Generate a unique hostname like "ecr-vm-a1b2c3"
     let hostname_suffix = format!("{:x}", (std::process::id() as u64)
         .wrapping_mul(std::time::SystemTime::now()
@@ -72,14 +75,14 @@ pub fn launch_qemu(config: QemuConfig) -> Result<()> {
     let kernel_append = if let Some(ref cmd) = config.command {
         let cmd_str = cmd.join(" ");
         format!(
-            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec sh </dev/ttyS0 >/dev/ttyS0 2>&1 -c {}'\"",
-            hostname, hostname, cmd_str
+            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec {} </dev/ttyS0 >/dev/ttyS0 2>&1 -c {}'\"",
+            hostname, hostname, shell, cmd_str
         )
     } else {
         // Default to interactive shell with proper job control
         format!(
-            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec sh </dev/ttyS0 >/dev/ttyS0 2>&1'\"",
-            hostname, hostname
+            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec {} </dev/ttyS0 >/dev/ttyS0 2>&1'\"",
+            hostname, hostname, shell
         )
     };