feat: set unique hostname in QEMU VM mode

Generate hostname like "ecr-vm-<hex>" at boot time via kernel command
line, leaving initramfs cacheable. Set via echo to /etc/hostname and
hostname command before spawning shell.

src/qemu_vm.rs

@@ -54,21 +54,33 @@ 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)
     ))?;
 
+    // Generate a unique hostname like "ecr-vm-a1b2c3"
+    let hostname_suffix = format!("{:x}", (std::process::id() as u64)
+        .wrapping_mul(std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .unwrap_or_default()
+            .as_nanos() as u64) % 0x1000000);  // 6 hex chars
+    let hostname = format!("ecr-vm-{}", hostname_suffix);
+    
     // Build kernel command line
     // For initramfs boot, use rdinit= instead of init=
     // No root= needed as initramfs becomes the rootfs
     // 'quiet' suppresses kernel log messages for a cleaner console
     // Use setsid with redirected stdio to get proper job control
     // /dev/ttyS0 is created in the initramfs for serial console
+    // Set hostname before spawning shell
     let kernel_append = if let Some(ref cmd) = config.command {
         let cmd_str = cmd.join(" ");
         format!(
-            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"setsid sh -c 'exec sh </dev/ttyS0 >/dev/ttyS0 2>&1 -c {}'\"",
-            cmd_str
+            "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
         )
     } else {
         // Default to interactive shell with proper job control
-        "console=ttyS0 quiet rdinit=/bin/sh -- -c \"setsid sh -c 'exec sh </dev/ttyS0 >/dev/ttyS0 2>&1'\"".to_string()
+        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
+        )
     };
 
     veprintln!("Launching QEMU: {}", qemu_bin);