feat: improve error diagnostics for native module failures

[dwmkerr]

Problem

When node-pty fails to load (e.g., missing prebuilt binaries), users see a cryptic error:

Error: posix_spawnp failed.

No context on what went wrong or how to fix it.

Proposed Solutions

1. Startup diagnostics

Wrap node-pty import with helpful error handling:

let pty;
try {
  pty = await import('node-pty');
} catch (err) {
  console.error(`[shellwright] Failed to load node-pty native module.`);
  console.error(`[shellwright] This usually means compilation failed during install.`);
  console.error(`[shellwright] Try: npm install -g @dwmkerr/shellwright`);
  console.error(`[shellwright] Or install build tools: xcode-select --install (macOS)`);
  console.error(`[shellwright] Original error:`, err.message);
  process.exit(1);
}

2. Add --verbose flag

shellwright --verbose

Outputs:

• Node version
• Platform/arch
• node-pty binding path
• Any load errors

3. Document npx debugging

Add to README troubleshooting:

# See npx installation logs
npm_config_loglevel=verbose npx -y @dwmkerr/shellwright

# Or check npm cache
ls ~/.npm/_npx/*/node_modules/node-pty/build/

4. Health check endpoint (HTTP mode)

GET /health → { "status": "ok", "node_pty": "loaded", "version": "0.1.4" }

Acceptance Criteria

• Startup catches node-pty load failures with actionable message
• --verbose flag shows diagnostic info
• README documents how to debug installation issues