docs(cli): add System Prompt Override (GEMINI_SYSTEM_MD)

[ashmod]

TLDR

Adds documentation for GEMINI_SYSTEM_MD to docs/cli

Linked issues / bugs

Closes #9500

[gemini-code-assist[bot]]

Summary of Changes

Hello @ashmod, I'm Gemini Code Assist[^1]! 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 introduces and documents a powerful new feature for the Gemini CLI: the ability to override the default system prompt using a custom Markdown file via the GEMINI_SYSTEM_MD environment variable. This enhancement provides advanced users with greater control over the CLI's behavior and persona, allowing for highly customized, project-specific instructions. Additionally, it documents a mechanism (GEMINI_WRITE_SYSTEM_MD) to export the default prompt, facilitating easier customization.

Highlights

• System Prompt Override (GEMINI_SYSTEM_MD): Documentation for the GEMINI_SYSTEM_MD environment variable has been added, enabling users to replace the Gemini CLI's built-in system prompt with content from a custom Markdown file.
• Exporting System Prompt (GEMINI_WRITE_SYSTEM_MD): The GEMINI_WRITE_SYSTEM_MD environment variable is now documented, allowing users to export the current built-in system prompt to a file for review or modification.
• Documentation Updates: New and updated documentation files (docs/core/system-prompt.md, docs/cli/configuration.md, docs/core/index.md, docs/index.md) have been added to comprehensively cover the system prompt override feature.

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[ashmod]

Hi there! This is great. I made a few tiny comments from a documentation perspective, mostly because some documents have changed places. Will still need review by eng.

Thanks for the review! I've addressed all the changes, lmk if I missed anything or if anything else needs to be modified.

[ashmod]

@jkcinouye I've updated this to remove conflicts with the base. Could you please check if everything is good to go?

[jkcinouye]

Hi there! I'm sorry for letting this slip - bear with me a bit. There's just one minor change, this would really go into /cli/ not /core/ (core is mostly for under the hood engine work, cli is for anything the user would interface with). Do you think you could update that? Otherwise I think this is GTG.

[ashmod]

Hi, the new docs now point to cli/

[ashmod]

Fixed the lint/broken link. CI should pass now.