feat(wave3): add @mosaic/cli — unified mosaic CLI entry point

- Root Commander program with version + description
- Subcommand groups: coord, prdy, queue, quality-rails
- Single `mosaic` binary entry point
- Depends on all @mosaic/* workspace packages

packages/cli/src/commands/prdy.ts

@@ -0,0 +1,13 @@
+import { Command } from 'commander';
+import pc from 'picocolors';
+
+// TODO(wave3): Replace this temporary shim once @mosaic/prdy lands in main.
+export function registerPrdyCommand(program: Command): void {
+  program
+    .command('prdy')
+    .description('PRD workflow commands')
+    .action(() => {
+      console.error(pc.yellow('@mosaic/prdy CLI is not available in this workspace yet.'));
+      process.exitCode = 1;
+    });
+}