fix: stub react-devtools-core for bun compile

Ink statically imports react-devtools-core (only used when DEV=true).
With --external, bun compile leaves a runtime require that fails in the
standalone binary. Instead, provide a no-op stub that bun bundles inline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/stubs/react-devtools-core/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "react-devtools-core",
+  "version": "0.0.0",
+  "main": "index.js",
+  "type": "module"
+}