Phase 4: remote catalog + banger kernel pull

Introduces the headline feature of the kernel catalog: pulling a kernel
bundle over HTTP without any local build step.

Catalog format (internal/kernelcat/catalog.go):
 - Catalog { Version, Entries } + CatEntry { Name, Distro, Arch,
   KernelVersion, TarballURL, TarballSHA256, SizeBytes, Description }.
 - catalog.json is embedded via go:embed and ships with each banger
   binary. It starts empty (Phase 5's CI pipeline will populate it).
 - Lookup(name) returns the matching entry or os.ErrNotExist.

Fetch (internal/kernelcat/fetch.go):
 - HTTP GET with streaming SHA256 over the response body.
 - zstd-decode (github.com/klauspost/compress/zstd) -> tar extract into
   <kernelsDir>/<name>/.
 - Hardens against path-traversal tarball entries (members whose
   normalised path escapes the target dir, and unsafe symlink
   targets) and sha256-mismatch downloads; any failure removes the
   partially-populated target dir.
 - Regular files, directories, and safe symlinks are supported; other
   tar types (hardlinks, devices, fifos) are silently skipped.
 - After extraction, recomputes sha256 over the on-disk vmlinux and
   writes the manifest with Source="pull:<url>".

Daemon methods (internal/daemon/kernels.go):
 - KernelPull(ctx, {Name, Force}) - lookup in embedded catalog, refuse
   overwrite unless Force, delegate to kernelcat.Fetch.
 - KernelCatalog(ctx) - return the embedded catalog annotated per-entry
   with whether it has been pulled locally.

RPC: kernel.pull, kernel.catalog dispatch cases.

CLI:
 - `banger kernel pull <name> [--force]`.
 - `banger kernel list --available` prints the catalog with a
   pulled/available STATE column and a human-readable size.

Tests: fetch round-trip (extract + manifest + sha256), sha256 mismatch
rejection with cleanup, missing-vmlinux rejection, path-traversal
rejection, HTTP error propagation, catalog parsing, lookup,
pulled-status reconciliation. All 20 packages green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/cli/banger.go

@@ -1600,10 +1600,33 @@ func newKernelCommand() *cobra.Command {
 		newKernelShowCommand(),
 		newKernelRmCommand(),
 		newKernelImportCommand(),
+		newKernelPullCommand(),
 	)
 	return cmd
 }
 
+func newKernelPullCommand() *cobra.Command {
+	var force bool
+	cmd := &cobra.Command{
+		Use:   "pull <name>",
+		Short: "Download a cataloged kernel bundle",
+		Args:  exactArgsUsage(1, "usage: banger kernel pull <name> [--force]"),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			layout, _, err := ensureDaemon(cmd.Context())
+			if err != nil {
+				return err
+			}
+			result, err := rpc.Call[api.KernelShowResult](cmd.Context(), layout.SocketPath, "kernel.pull", api.KernelPullParams{Name: args[0], Force: force})
+			if err != nil {
+				return err
+			}
+			return printJSON(cmd.OutOrStdout(), result.Entry)
+		},
+	}
+	cmd.Flags().BoolVar(&force, "force", false, "re-pull even if already present")
+	return cmd
+}
+
 func newKernelImportCommand() *cobra.Command {
 	var params api.KernelImportParams
 	cmd := &cobra.Command{
@@ -1639,15 +1662,23 @@ func newKernelImportCommand() *cobra.Command {
 }
 
 func newKernelListCommand() *cobra.Command {
-	return &cobra.Command{
+	var available bool
+	cmd := &cobra.Command{
 		Use:   "list",
-		Short: "List locally available kernels",
-		Args:  noArgsUsage("usage: banger kernel list"),
+		Short: "List kernels (local by default, or --available for the catalog)",
+		Args:  noArgsUsage("usage: banger kernel list [--available]"),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			layout, _, err := ensureDaemon(cmd.Context())
 			if err != nil {
 				return err
 			}
+			if available {
+				result, err := rpc.Call[api.KernelCatalogResult](cmd.Context(), layout.SocketPath, "kernel.catalog", api.Empty{})
+				if err != nil {
+					return err
+				}
+				return printKernelCatalogTable(cmd.OutOrStdout(), result.Entries)
+			}
 			result, err := rpc.Call[api.KernelListResult](cmd.Context(), layout.SocketPath, "kernel.list", api.Empty{})
 			if err != nil {
 				return err
@@ -1655,6 +1686,8 @@ func newKernelListCommand() *cobra.Command {
 			return printKernelListTable(cmd.OutOrStdout(), result.Entries)
 		},
 	}
+	cmd.Flags().BoolVar(&available, "available", false, "show the built-in catalog (with pulled/available status) instead of local entries")
+	return cmd
 }
 
 func newKernelShowCommand() *cobra.Command {
@@ -1717,6 +1750,53 @@ func printKernelListTable(out anyWriter, entries []api.KernelEntry) error {
 	return w.Flush()
 }
 
+func printKernelCatalogTable(out anyWriter, entries []api.KernelCatalogEntry) error {
+	w := tabwriter.NewWriter(out, 0, 8, 2, ' ', 0)
+	if _, err := fmt.Fprintln(w, "NAME\tDISTRO\tARCH\tKERNEL\tSIZE\tSTATE"); err != nil {
+		return err
+	}
+	for _, entry := range entries {
+		state := "available"
+		if entry.Pulled {
+			state = "pulled"
+		}
+		if _, err := fmt.Fprintf(
+			w,
+			"%s\t%s\t%s\t%s\t%s\t%s\n",
+			entry.Name,
+			dashIfEmpty(entry.Distro),
+			dashIfEmpty(entry.Arch),
+			dashIfEmpty(entry.KernelVersion),
+			humanSize(entry.SizeBytes),
+			state,
+		); err != nil {
+			return err
+		}
+	}
+	return w.Flush()
+}
+
+func humanSize(bytes int64) string {
+	if bytes <= 0 {
+		return "-"
+	}
+	const (
+		kib = 1024
+		mib = 1024 * kib
+		gib = 1024 * mib
+	)
+	switch {
+	case bytes >= gib:
+		return fmt.Sprintf("%.1fGiB", float64(bytes)/float64(gib))
+	case bytes >= mib:
+		return fmt.Sprintf("%.1fMiB", float64(bytes)/float64(mib))
+	case bytes >= kib:
+		return fmt.Sprintf("%.1fKiB", float64(bytes)/float64(kib))
+	default:
+		return fmt.Sprintf("%dB", bytes)
+	}
+}
+
 func dashIfEmpty(s string) string {
 	if strings.TrimSpace(s) == "" {
 		return "-"

internal/cli/cli_test.go

@@ -75,7 +75,7 @@ func TestKernelCommandExposesSubcommands(t *testing.T) {
 	for _, sub := range kernel.Commands() {
 		names = append(names, sub.Name())
 	}
-	want := []string{"import", "list", "rm", "show"}
+	want := []string{"import", "list", "pull", "rm", "show"}
 	if !reflect.DeepEqual(names, want) {
 		t.Fatalf("kernel subcommands = %v, want %v", names, want)
 	}