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/paths/paths_test.go

@@ -0,0 +1,69 @@
+package paths
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestResolveRuntimeDirPrefersEnv(t *testing.T) {
+	t.Setenv("BANGER_RUNTIME_DIR", "/env/runtime")
+
+	if got := ResolveRuntimeDir("/config/runtime", "/deprecated/repo"); got != "/env/runtime" {
+		t.Fatalf("ResolveRuntimeDir() = %q, want /env/runtime", got)
+	}
+}
+
+func TestResolveRuntimeDirUsesInstalledLayout(t *testing.T) {
+	root := t.TempDir()
+	runtimeDir := filepath.Join(root, "lib", "banger")
+	createRuntimeBundle(t, runtimeDir)
+
+	origExecutablePath := executablePath
+	executablePath = func() (string, error) {
+		return filepath.Join(root, "bin", "banger"), nil
+	}
+	t.Cleanup(func() {
+		executablePath = origExecutablePath
+	})
+
+	if got := ResolveRuntimeDir("", ""); got != runtimeDir {
+		t.Fatalf("ResolveRuntimeDir() = %q, want %q", got, runtimeDir)
+	}
+}
+
+func TestResolveRuntimeDirUsesExecutableDirectoryBundle(t *testing.T) {
+	root := t.TempDir()
+	createRuntimeBundle(t, root)
+
+	origExecutablePath := executablePath
+	executablePath = func() (string, error) {
+		return filepath.Join(root, "banger"), nil
+	}
+	t.Cleanup(func() {
+		executablePath = origExecutablePath
+	})
+
+	if got := ResolveRuntimeDir("", ""); got != root {
+		t.Fatalf("ResolveRuntimeDir() = %q, want %q", got, root)
+	}
+}
+
+func createRuntimeBundle(t *testing.T, runtimeDir string) {
+	t.Helper()
+	for _, rel := range []string{
+		"firecracker",
+		"customize.sh",
+		"packages.apt",
+		"rootfs-docker.ext4",
+		"wtf/root/boot/vmlinux-6.8.0-94-generic",
+	} {
+		path := filepath.Join(runtimeDir, rel)
+		if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+			t.Fatalf("mkdir %s: %v", filepath.Dir(path), err)
+		}
+		if err := os.WriteFile(path, []byte("test"), 0o644); err != nil {
+			t.Fatalf("write %s: %v", path, err)
+		}
+	}
+}