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/kernels.go

@@ -0,0 +1,67 @@
+package daemon
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"time"
+
+	"banger/internal/api"
+	"banger/internal/kernelcat"
+)
+
+func (d *Daemon) KernelList(_ context.Context) (api.KernelListResult, error) {
+	entries, err := kernelcat.ListLocal(d.layout.KernelsDir)
+	if err != nil {
+		return api.KernelListResult{}, err
+	}
+	result := api.KernelListResult{Entries: make([]api.KernelEntry, 0, len(entries))}
+	for _, entry := range entries {
+		result.Entries = append(result.Entries, kernelEntryToAPI(entry))
+	}
+	return result, nil
+}
+
+func (d *Daemon) KernelShow(_ context.Context, name string) (api.KernelEntry, error) {
+	entry, err := kernelcat.ReadLocal(d.layout.KernelsDir, name)
+	if err != nil {
+		return api.KernelEntry{}, kernelNotFoundIfMissing(name, err)
+	}
+	return kernelEntryToAPI(entry), nil
+}
+
+func (d *Daemon) KernelDelete(_ context.Context, name string) error {
+	if err := kernelcat.ValidateName(name); err != nil {
+		return err
+	}
+	return kernelcat.DeleteLocal(d.layout.KernelsDir, name)
+}
+
+func kernelEntryToAPI(entry kernelcat.Entry) api.KernelEntry {
+	importedAt := ""
+	if !entry.ImportedAt.IsZero() {
+		importedAt = entry.ImportedAt.UTC().Format(time.RFC3339)
+	}
+	return api.KernelEntry{
+		Name:          entry.Name,
+		Distro:        entry.Distro,
+		Arch:          entry.Arch,
+		KernelVersion: entry.KernelVersion,
+		SHA256:        entry.SHA256,
+		Source:        entry.Source,
+		ImportedAt:    importedAt,
+		KernelPath:    entry.KernelPath,
+		InitrdPath:    entry.InitrdPath,
+		ModulesDir:    entry.ModulesDir,
+	}
+}
+
+func kernelNotFoundIfMissing(name string, err error) error {
+	if err == nil {
+		return nil
+	}
+	if os.IsNotExist(err) {
+		return fmt.Errorf("kernel %q not found", name)
+	}
+	return err
+}