Create authoring assistant example application

[geoffjay]

Summary

Create a purpose-built example at examples/assistant/app.xml that demonstrates the full generate-preview-iterate loop: a chat panel with Nemo-specific system prompt, a code editor showing the generated config, and a launch preview button.

Context

This example becomes the recommended entry point for new Nemo developers. It combines:

• A <chat> panel with a Nemo-specific system prompt that includes the full component schema
• A <code-editor> pane displaying the current generated config
• A "Launch Preview" button that writes the code editor content to a temp file and runs the Nemo app
• A simplified component reference panel (leveraging the storybook from the Component Storybook epic)

The application should feel like pair-programming with an AI that knows Nemo.

Acceptance Criteria

• Example directory created at examples/assistant/ with:
  • app.xml — main config
  • scripts/handlers.rhai — event handlers for launch, edit, navigation
  • README.md — setup and usage instructions
• Layout: horizontal split with chat panel (left) and preview panel (right)
• Chat panel initialized with Nemo-specific system prompt (from generated schema reference)
• Code editor pane displays the most recent generated config with syntax highlighting (language="xml")
• "Launch Preview" button writes editor content to a temp XML file in $TMPDIR/nemo-preview/
• "Launch Preview" invokes the Nemo runtime to load and render the temp file
• Component reference panel (right sidebar or bottom section) shows a compact list of available components from the registry
• Panel includes search/filter input for components
• README documents: prerequisites (LLM API key), how to launch, workflow description
• Handles the on-response event from <chat> to update the code editor

Relevant Files

• examples/ — existing examples for reference
• crates/nemo/src/components/code_editor.rs — code editor component
• crates/nemo/src/runtime.rs — NemoRuntime for temp file loading

Stack Base

• Stack on: main
• Blocked by: Add <chat> component type for multi-turn LLM conversations #58 (chat component), Add nemo generate CLI subcommand for NL-to-config generation #57 (nemo generate subcommand) — needs both the chat UI and the generate pipeline