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/daemon/daemon.go

@@ -527,6 +527,22 @@ func (d *Daemon) dispatch(ctx context.Context, req rpc.Request) rpc.Response {
 		}
 		image, err := d.DeleteImage(ctx, params.IDOrName)
 		return marshalResultOrError(api.ImageShowResult{Image: image}, err)
+	case "kernel.list":
+		return marshalResultOrError(d.KernelList(ctx))
+	case "kernel.show":
+		params, err := rpc.DecodeParams[api.KernelRefParams](req)
+		if err != nil {
+			return rpc.NewError("bad_request", err.Error())
+		}
+		entry, err := d.KernelShow(ctx, params.Name)
+		return marshalResultOrError(api.KernelShowResult{Entry: entry}, err)
+	case "kernel.delete":
+		params, err := rpc.DecodeParams[api.KernelRefParams](req)
+		if err != nil {
+			return rpc.NewError("bad_request", err.Error())
+		}
+		err = d.KernelDelete(ctx, params.Name)
+		return marshalResultOrError(api.Empty{}, err)
 	default:
 		return rpc.NewError("unknown_method", req.Method)
 	}