feat: add go support to upgrade command and update docs

[allen-stephen]

Summary

• Add Go project support to the upgrade command
• Consolidate duplicate language-handling code into shared utils/language.py module
• Add CLI reference documentation for extract and upgrade commands
• Update CLI index and sidebar navigation
• Add "Keeping Your Project Up-to-Date" section to development guide
• Add upgrade command mention to generated project READMEs
• Document both commands in llm.txt for coding agent consumption

[gemini-code-assist]

Summary of Changes

Hello @allen-stephen, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the agent-starter-pack's documentation by introducing detailed guides for the new extract and upgrade CLI commands. These additions ensure that users have clear instructions on how to manage project scaffolding and keep their projects current with the latest agent-starter-pack versions, thereby improving the overall user experience and project maintainability.

Highlights

• New CLI Documentation: Comprehensive documentation has been added for the new extract and upgrade CLI commands, detailing their usage, arguments, options, and operational logic.
• Navigation and Index Updates: The CLI index and sidebar navigation in the documentation have been updated to include direct links to the newly added extract and upgrade command documentation pages.
• Project README Integration: The README.md files for both Go and Python base templates now include a new 'Keeping Up-to-Date' section, guiding users on how to use the upgrade command.
• Development Guide Enhancement: A new section titled 'Keeping Your Project Up-to-Date' has been added to the development guide, explaining the purpose and benefits of the upgrade command.
• Coding Agent Documentation: Detailed descriptions of both the extract and upgrade commands, including their functionalities and examples, have been added to llm.txt for consumption by coding agents.

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[gemini-code-assist]

Code Review

This pull request adds extensive documentation for the new extract and upgrade CLI commands. The documentation is well-written and comprehensive. My review focuses on improving clarity and ensuring accuracy, particularly regarding language support for these new commands.

I've identified a critical issue where the upgrade command is documented for Go projects, but the implementation is Python-only. I've also suggested several high and medium-severity improvements to clarify language-specific behaviors in the documentation for both extract and upgrade to prevent user confusion.

Overall, great work on the detailed documentation.