Phase 1: local kernel catalog scaffolding

Introduces a read/write kernel catalog on disk without any network
dependency, so later phases (image register --kernel-ref, import, pull)
can build on a working foundation.

Layout: adds KernelsDir to paths.Layout, ensured under
~/.local/state/banger/kernels/. Each cataloged kernel lives at
<KernelsDir>/<name>/ with a manifest.json alongside vmlinux and optional
initrd.img / modules/.

New internal/kernelcat package owns the disk format:
- Entry (Name, Distro, Arch, KernelVersion, SHA256, Source, ImportedAt)
- ValidateName (alphanumeric + dots/hyphens/underscores, no traversal)
- ReadLocal / ListLocal / WriteLocal / DeleteLocal
- SumFile helper

The daemon exposes three RPC methods dispatched in daemon.go:
kernel.list, kernel.show, kernel.delete. Implementations live in a new
internal/daemon/kernels.go and are thin wrappers over kernelcat using
d.layout.KernelsDir.

CLI: new top-level `banger kernel` with list / show / rm subcommands
mirroring the image-command pattern (ensureDaemon, RPC call, table or
JSON output). No sudo required — kernel ops are user-space only.

Users can now manually populate ~/.local/state/banger/kernels/<name>/
and see it via `banger kernel list`. Phase 2 wires --kernel-ref into
image register; Phase 3 adds `banger kernel import`; Phase 4 adds
remote pulls.

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

internal/cli/cli_test.go

@@ -19,6 +19,8 @@ import (
 	"banger/internal/model"
 	"banger/internal/system"
 	"banger/internal/toolingplan"
+
+	"github.com/spf13/cobra"
 )
 
 func TestNewBangerCommandHasExpectedSubcommands(t *testing.T) {
@@ -27,7 +29,7 @@ func TestNewBangerCommandHasExpectedSubcommands(t *testing.T) {
 	for _, sub := range cmd.Commands() {
 		names = append(names, sub.Name())
 	}
-	want := []string{"daemon", "doctor", "image", "internal", "ps", "version", "vm"}
+	want := []string{"daemon", "doctor", "image", "internal", "kernel", "ps", "version", "vm"}
 	if !reflect.DeepEqual(names, want) {
 		t.Fatalf("subcommands = %v, want %v", names, want)
 	}
@@ -57,6 +59,28 @@ func TestVersionCommandPrintsBuildInfo(t *testing.T) {
 	}
 }
 
+func TestKernelCommandExposesSubcommands(t *testing.T) {
+	cmd := NewBangerCommand()
+	var kernel *cobra.Command
+	for _, sub := range cmd.Commands() {
+		if sub.Name() == "kernel" {
+			kernel = sub
+			break
+		}
+	}
+	if kernel == nil {
+		t.Fatalf("kernel command missing from root")
+	}
+	names := []string{}
+	for _, sub := range kernel.Commands() {
+		names = append(names, sub.Name())
+	}
+	want := []string{"list", "rm", "show"}
+	if !reflect.DeepEqual(names, want) {
+		t.Fatalf("kernel subcommands = %v, want %v", names, want)
+	}
+}
+
 func TestLegacyRemovedCommandIsRejected(t *testing.T) {
 	cmd := NewBangerCommand()
 	cmd.SetArgs([]string{"tui"})