Make installed banger self-contained

Fix the misleading make install path where banger and bangerd still depended on a repo checkout for Firecracker, guest artifacts, image builds, and the SSH key.

Replace repo-root inference with an explicit runtime bundle model: resolve a runtime_dir from env/config/install layout, derive concrete artifact paths from it, and update the daemon, CLI, and image-build flow to use those paths. Keep repo_root only as an explicit compatibility alias instead of auto-detecting it.

Teach customize.sh to run from a read-only bundled runtime tree while writing transient state under XDG/BANGER_STATE_DIR, and make make install copy the runtime assets into PREFIX/lib/banger so installed binaries stay usable outside the repo.

Validate with go test ./..., make build, bash -n customize.sh, and make install DESTDIR=/tmp/banger-install PREFIX=/usr. An out-of-repo installed-binary smoke test was attempted, but this sandbox blocked bangerd from binding its Unix socket (setsockopt: operation not permitted).

internal/daemon/daemon_test.go

@@ -0,0 +1,54 @@
+package daemon
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"banger/internal/model"
+	"banger/internal/store"
+)
+
+func TestEnsureDefaultImageUsesConfiguredDefaultRootfs(t *testing.T) {
+	dir := t.TempDir()
+	rootfs := filepath.Join(dir, "rootfs-docker.ext4")
+	kernel := filepath.Join(dir, "vmlinux")
+	for _, path := range []string{rootfs, kernel} {
+		if err := os.WriteFile(path, []byte("test"), 0o644); err != nil {
+			t.Fatalf("write %s: %v", path, err)
+		}
+	}
+
+	db, err := store.Open(filepath.Join(dir, "state.db"))
+	if err != nil {
+		t.Fatalf("open store: %v", err)
+	}
+	t.Cleanup(func() {
+		_ = db.Close()
+	})
+
+	d := &Daemon{
+		config: model.DaemonConfig{
+			DefaultImageName: "default",
+			DefaultRootfs:    rootfs,
+			DefaultKernel:    kernel,
+		},
+		store: db,
+	}
+
+	if err := d.ensureDefaultImage(context.Background()); err != nil {
+		t.Fatalf("ensureDefaultImage: %v", err)
+	}
+
+	image, err := db.GetImageByName(context.Background(), "default")
+	if err != nil {
+		t.Fatalf("GetImageByName: %v", err)
+	}
+	if image.RootfsPath != rootfs {
+		t.Fatalf("RootfsPath = %q, want %q", image.RootfsPath, rootfs)
+	}
+	if image.KernelPath != kernel {
+		t.Fatalf("KernelPath = %q, want %q", image.KernelPath, kernel)
+	}
+}