feat(cli): scaffold nc CLI with list-groups command

Adds a transport-agnostic CLI control plane shared between three eventual
callers (host shell, Claude in project, container agent) — though only the
host-side socket transport is wired in this commit. Container DB transport
and approval flow land alongside the first risky command.

- src/cli/frame.ts:        wire format (RequestFrame, ResponseFrame, CallerContext)
- src/cli/registry.ts:     command registry with RiskClass
- src/cli/dispatch.ts:     transport-agnostic dispatcher
- src/cli/transport.ts:    Transport interface
- src/cli/socket-client.ts: SocketTransport against data/nc.sock
- src/cli/socket-server.ts: host-side listener (chmod 0600, line-delimited JSON)
- src/cli/format.ts:       human table / --json output modes
- src/cli/client.ts:       `nc` argv -> frame -> transport -> stdout
- src/cli/commands/list-groups.ts: first command (riskClass: safe)
- bin/nc:                  bash launcher (resolves project root via symlink)
- src/index.ts:            start/stop server + import command barrel

`data/nc.sock` is intentionally separate from `data/cli.sock` (which the
existing chat-style channel adapter still owns).

Verified end-to-end: `nc list-groups`, `nc list groups`, `--json`,
unknown-command error, host-down ENOENT message with start instructions.
typecheck clean; eslint reports only the same `no-catch-all` warnings the
rest of the codebase has.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/cli/registry.ts

@@ -0,0 +1,36 @@
+/**
+ * Command registry — single source of truth for what `nc` can do.
+ *
+ * Each command file under `commands/` calls `register()` at top level,
+ * and `commands/index.ts` imports them all for side effects so the
+ * registry is populated before the host's CLI server accepts connections.
+ */
+import type { CallerContext } from './frame.js';
+
+export type RiskClass = 'safe' | 'requires-admin' | 'requires-owner';
+
+export type CommandDef<TArgs = unknown, TData = unknown> = {
+  name: string;
+  description: string;
+  riskClass: RiskClass;
+  /** Validates `frame.args` and produces the typed handler input. Throws on invalid. */
+  parseArgs: (raw: Record<string, unknown>) => TArgs;
+  handler: (args: TArgs, ctx: CallerContext) => Promise<TData>;
+};
+
+const registry = new Map<string, CommandDef>();
+
+export function register<TArgs, TData>(def: CommandDef<TArgs, TData>): void {
+  if (registry.has(def.name)) {
+    throw new Error(`CLI command "${def.name}" already registered`);
+  }
+  registry.set(def.name, def as CommandDef);
+}
+
+export function lookup(name: string): CommandDef | undefined {
+  return registry.get(name);
+}
+
+export function listCommands(): CommandDef[] {
+  return [...registry.values()].sort((a, b) => a.name.localeCompare(b.name));
+}