Add experimental Void guest workflow and vsock agent

Make iterating on a Firecracker-friendly Void guest practical without replacing the Debian default image path.

Add local Void rootfs build/register/verify plumbing, a language-agnostic dev package baseline, and guest SSH/work-disk hardening so new images use the runtime bundle key, keep a normal root bash environment, and repair stale nested /root layouts on restart.

Replace the guest PING/PONG responder with an HTTP /healthz agent over vsock, rename the runtime bundle and config surface from ping helper to agent while still accepting the legacy keys, and route the post-SSH reminder through the new vm.health path.

Validated with GOCACHE=/tmp/banger-gocache go test ./..., make build, bash -n customize.sh make-rootfs-void.sh, and git diff --check.

internal/api/types.go

@@ -63,6 +63,11 @@ type VMSSHResult struct {
 	GuestIP string `json:"guest_ip"`
 }
 
+type VMHealthResult struct {
+	Name    string `json:"name"`
+	Healthy bool   `json:"healthy"`
+}
+
 type VMPingResult struct {
 	Name  string `json:"name"`
 	Alive bool   `json:"alive"`
@@ -78,6 +83,17 @@ type ImageBuildParams struct {
 	Docker     bool   `json:"docker,omitempty"`
 }
 
+type ImageRegisterParams struct {
+	Name         string `json:"name,omitempty"`
+	RootfsPath   string `json:"rootfs_path,omitempty"`
+	WorkSeedPath string `json:"work_seed_path,omitempty"`
+	KernelPath   string `json:"kernel_path,omitempty"`
+	InitrdPath   string `json:"initrd_path,omitempty"`
+	ModulesDir   string `json:"modules_dir,omitempty"`
+	PackagesPath string `json:"packages_path,omitempty"`
+	Docker       bool   `json:"docker,omitempty"`
+}
+
 type ImageRefParams struct {
 	IDOrName string `json:"id_or_name"`
 }