feat: support standalone browser mode overrides

[0xsline]

Summary

• add OPENCLI_BROWSER_MODE=extension|standalone to override the default browser mode selection
• add OPENCLI_MCP_NO_SANDBOX=1 support for Linux root / VPS Chromium runs
• document the new VPS / CI flow in the README and testing docs

Why

Today buildMcpArgs() always forces --extension outside CI. That works well on a local machine with the Playwright MCP Bridge extension, but it makes VPS and headless Linux setups awkward even when Chromium is already installed.

I hit two concrete issues while testing on a Debian VPS:

• Extension connection timeout when no MCP browser extension was present
• Chromium launch failures as root unless --no-sandbox was passed

This change keeps the current defaults intact, but adds an explicit escape hatch for standalone environments.

Testing

• npm test -- src/browser.test.ts
• npm run build

[jackwener]

It is currently supported, please refer to the latest documentation.