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/paths/paths.go

@@ -22,6 +22,7 @@ type Layout struct {
 	DaemonLog   string
 	VMsDir      string
 	ImagesDir   string
+	KernelsDir  string
 }
 
 func Resolve() (Layout, error) {
@@ -52,11 +53,12 @@ func Resolve() (Layout, error) {
 	layout.DaemonLog = filepath.Join(layout.StateDir, "bangerd.log")
 	layout.VMsDir = filepath.Join(layout.StateDir, "vms")
 	layout.ImagesDir = filepath.Join(layout.StateDir, "images")
+	layout.KernelsDir = filepath.Join(layout.StateDir, "kernels")
 	return layout, nil
 }
 
 func Ensure(layout Layout) error {
-	for _, dir := range []string{layout.ConfigDir, layout.StateDir, layout.CacheDir, layout.RuntimeDir, layout.VMsDir, layout.ImagesDir} {
+	for _, dir := range []string{layout.ConfigDir, layout.StateDir, layout.CacheDir, layout.RuntimeDir, layout.VMsDir, layout.ImagesDir, layout.KernelsDir} {
 		if err := os.MkdirAll(dir, 0o755); err != nil {
 			return err
 		}