Add live preview integration for VS Code extension

[geoffjay]

Summary

Integrate Nemo's dev watch mode with the VS Code extension so that saving a Nemo XML file automatically reloads the running preview. Also add a "Preview in Nemo" command that launches or focuses the Nemo window for the current config.

Context

The nemo dev --watch mode (from the Build and Installation epic, #10) watches for file changes and hot-reloads the application. The VS Code extension should surface this as a seamless preview experience:

• Launch the app in dev mode from VS Code
• Save → auto-reload the preview window
• Keep the preview window visible alongside the editor

This creates a tight feedback loop: edit XML → Ctrl+S → see changes instantly.

Acceptance Criteria

• "Preview in Nemo" VS Code command:
  • Saves current file
  • If no preview is running, launches nemo dev --watch --app-config <file> in a terminal
  • If preview is already running, focuses the Nemo window (platform-specific)
• nemo dev subcommand (from Build epic) must signal ready state so VS Code knows when to show preview
• Status bar indicator in VS Code showing preview state (running/stopped/error)
• "Stop Preview" command to terminate the dev server
• On file save during preview, VS Code sends a notification or the watcher picks up the change
• Preview terminal reused across saves (not spawning a new process each time)
• Error handling: if Nemo binary not found, show setup instructions

Relevant Files

• extensions/nemo-vscode/ — VS Code extension
• crates/nemo/src/args.rs — CLI (needs dev subcommand from Build epic)
• crates/nemo/src/main.rs — app entry point

Stack Base

• Stack on: main
• Blocked by: the Build and Installation epic (hot-reload / nemo dev --watch) and Create nemo-vscode VS Code extension #63 (nemo-vscode extension)