Grok Build 0.2.68 lands agent env var

While mostly a quality-of-life update, the ability to update MCP servers without restarting sessions significantly streamlines the developer feedback loop. Setting `GROK_AGENT=1` is a small but critical addition that allows host tools to programmatically identify when they are operated by the agent.

• –Dynamic MCP configuration reduces friction when switching tools or updating API keys during active coding sessions.
• –The `GROK_AGENT=1` environment variable enables host terminals to prevent infinite loops, restrict dangerous commands, or format output specifically for the LLM.
• –This release continues xAI's push to make Grok Build a first-class citizen in professional terminal environments, prioritizing agent-host integration.