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/firecracker/client.go

@@ -1,23 +1,19 @@
 package firecracker
 
 import (
-	"bufio"
 	"context"
-	"fmt"
 	"io"
 	"log/slog"
 	"os"
 	"os/exec"
 	"strings"
 	"sync"
-	"time"
 
 	sdk "github.com/firecracker-microvm/firecracker-go-sdk"
 	models "github.com/firecracker-microvm/firecracker-go-sdk/client/models"
-	sdkvsock "github.com/firecracker-microvm/firecracker-go-sdk/vsock"
 	"github.com/sirupsen/logrus"
 
-	"banger/internal/vsockping"
+	"banger/internal/vsockagent"
 )
 
 type MachineConfig struct {
@@ -212,37 +208,12 @@ func newLogger(base *slog.Logger) *logrus.Entry {
 	return logrus.NewEntry(logger)
 }
 
+func HealthVSock(ctx context.Context, logger *slog.Logger, socketPath string) error {
+	return vsockagent.Health(ctx, logger, socketPath)
+}
+
 func PingVSock(ctx context.Context, logger *slog.Logger, socketPath string) error {
-	conn, err := sdkvsock.DialContext(
-		ctx,
-		socketPath,
-		vsockping.Port,
-		sdkvsock.WithRetryTimeout(3*time.Second),
-		sdkvsock.WithRetryInterval(100*time.Millisecond),
-		sdkvsock.WithLogger(newLogger(logger)),
-	)
-	if err != nil {
-		return err
-	}
-	defer conn.Close()
-
-	if deadline, ok := ctx.Deadline(); ok {
-		_ = conn.SetDeadline(deadline)
-	} else {
-		_ = conn.SetDeadline(time.Now().Add(3 * time.Second))
-	}
-
-	if _, err := io.WriteString(conn, vsockping.RequestLine); err != nil {
-		return err
-	}
-	line, err := bufio.NewReader(conn).ReadString('\n')
-	if err != nil {
-		return err
-	}
-	if strings.TrimSpace(line) != strings.TrimSpace(vsockping.ResponseLine) {
-		return fmt.Errorf("unexpected vsock response %q", strings.TrimSpace(line))
-	}
-	return nil
+	return HealthVSock(ctx, logger, socketPath)
 }
 
 type slogHook struct {