docs(mcp): rewrite for current API + add per-project install warning

Update mcp.md to match the current tool surface (cancel, polymorphic
pauseAt, settleMs, plugins-only-on-start_browser, availableObjects,
WebElement auto-describe, traceFile per scenario) and warn against
running the MCP server from a global codeceptjs install.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: DavertMik