Add dynamic() support to prompt.form() plugin API

- prompt.form() inputs can now have dynamic() callbacks that update
  reactively when form values change (same pattern as auth/template plugins)
- Changed PromptFormRequest routing from one-shot to bidirectional events
- Added PromptFormResponse.done field to distinguish intermediate updates
- Added optional size (enum) to PromptFormRequest for dialog sizing
- Added optional rows to FormInputEditor for fixed height editors
- New httpsnippet plugin: generates code snippets with dynamic language
  and library selectors that update the code preview in real-time

package.json

@@ -12,6 +12,7 @@
     "packages/plugin-runtime-types",
     "plugins-external/mcp-server",
     "plugins-external/template-function-faker",
+    "plugins-external/httpsnippet",
     "plugins/action-copy-curl",
     "plugins/action-copy-grpcurl",
     "plugins/action-send-folder",