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/daemon/images_pull.go

@@ -75,7 +75,7 @@ func (d *Daemon) pullFromOCI(ctx context.Context, params api.ImagePullParams) (i
 		return model.Image{}, fmt.Errorf("image %q already exists (id=%s); pick a different --name or delete it first", imgName, existing.ID)
 	}
 
-	kernelPath, initrdPath, modulesDir, err := d.resolveKernelInputs(params.KernelRef, params.KernelPath, params.InitrdPath, params.ModulesDir)
+	kernelPath, initrdPath, modulesDir, err := d.resolveKernelInputs(ctx, params.KernelRef, params.KernelPath, params.InitrdPath, params.ModulesDir)
 	if err != nil {
 		return model.Image{}, err
 	}
@@ -181,7 +181,7 @@ func (d *Daemon) pullFromBundle(ctx context.Context, params api.ImagePullParams,
 	if kernelRef == "" && strings.TrimSpace(params.KernelPath) == "" {
 		kernelRef = strings.TrimSpace(entry.KernelRef)
 	}
-	kernelPath, initrdPath, modulesDir, err := d.resolveKernelInputs(kernelRef, params.KernelPath, params.InitrdPath, params.ModulesDir)
+	kernelPath, initrdPath, modulesDir, err := d.resolveKernelInputs(ctx, kernelRef, params.KernelPath, params.InitrdPath, params.ModulesDir)
 	if err != nil {
 		return model.Image{}, err
 	}