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/api/types.go

@@ -274,6 +274,31 @@ type ImageShowResult struct {
 	Image model.Image `json:"image"`
 }
 
+type KernelEntry struct {
+	Name          string `json:"name"`
+	Distro        string `json:"distro,omitempty"`
+	Arch          string `json:"arch,omitempty"`
+	KernelVersion string `json:"kernel_version,omitempty"`
+	SHA256        string `json:"sha256,omitempty"`
+	Source        string `json:"source,omitempty"`
+	ImportedAt    string `json:"imported_at,omitempty"`
+	KernelPath    string `json:"kernel_path,omitempty"`
+	InitrdPath    string `json:"initrd_path,omitempty"`
+	ModulesDir    string `json:"modules_dir,omitempty"`
+}
+
+type KernelListResult struct {
+	Entries []KernelEntry `json:"entries"`
+}
+
+type KernelRefParams struct {
+	Name string `json:"name"`
+}
+
+type KernelShowResult struct {
+	Entry KernelEntry `json:"entry"`
+}
+
 type SudoStatus struct {
 	Available bool   `json:"available"`
 	Command   string `json:"command,omitempty"`