Merge grok cli main into original repo

[MetcalfeTom]

What does this PR do?

This PR merges the latest changes from the main branch of the original superagent-ai/grok-cli repository into this branch.

Key updates from the upstream merge include:

• Default Model Update: The default AI model has been updated to grok-code-fast-1 across the CLI, settings, and documentation.
• Initial Message Support: The CLI now accepts an initial message as a positional argument, allowing users to start a conversation or execute a prompt directly upon launch (e.g., grok "explain this code").
• Minor Tweaks and Fixes: Incorporates various small improvements and bug fixes from the upstream repository.

Fixes #

Checklist

• I tested my changes
• I reviewed my own code

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