vm create: auto-pull image and kernel from catalogs if missing

One-command sandbox: `banger vm run` on a fresh host now Just Works.
No prior `banger image pull` or `banger kernel pull` needed.

Changes:

- Default `default_image_name` flips from "default" to "debian-bookworm"
  so the golden image is the implicit target when `--image` is omitted.
- `CreateVM` resolves the image via a new `findOrAutoPullImage`: try
  the local store first, and on miss fall back to the embedded imagecat
  catalog + auto-pull. Emits a vm-create progress stage so the user
  sees "pulling from image catalog" in the create output.
- `resolveKernelInputs` gains context + the same pattern via
  `readOrAutoPullKernel`: try the local kernelcat, and on miss look up
  the embedded kernelcat and auto-pull. Fires whenever a bundle's
  manifest references a kernel the user hasn't pulled yet, not just
  during image pull — any CreateVM with an image that needs a kernel
  not yet local will resolve it.
- `--image` help text updated on both `vm run` and `vm create`.

Six tests cover local-hit-no-pull, auto-pull-on-miss, not-in-catalog
error propagation, and a non-ENOENT kernel read error does NOT trigger
a misleading "not in catalog" claim.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -46,7 +46,7 @@ func Load(layout paths.Layout) (model.DaemonConfig, error) {
 		CIDR:                model.DefaultCIDR,
 		TapPoolSize:         4,
 		DefaultDNS:          model.DefaultDNS,
-		DefaultImageName:    "default",
+		DefaultImageName:    "debian-bookworm",
 	}
 
 	var file fileConfig

internal/config/config_test.go

@@ -35,8 +35,8 @@ func TestLoadDefaultsResolveFirecrackerAndGenerateSSHKey(t *testing.T) {
 			t.Fatalf("stat %s: %v", path, err)
 		}
 	}
-	if cfg.DefaultImageName != "default" {
-		t.Fatalf("DefaultImageName = %q, want default", cfg.DefaultImageName)
+	if cfg.DefaultImageName != "debian-bookworm" {
+		t.Fatalf("DefaultImageName = %q, want debian-bookworm", cfg.DefaultImageName)
 	}
 	if cfg.WebListenAddr != "127.0.0.1:7777" {
 		t.Fatalf("WebListenAddr = %q", cfg.WebListenAddr)