Fallback to ~/.gemini/system.md when .gemini/system.md is missing and GEMINI_SYSTEM_MD is set to 1 or true. #15549
Description
What would you like to be added?
When GEMINI_SYSTEM_MD is set to 1 or true, use .gemini/system.md if it exists in the project; otherwise, fall back to $HOME/.gemini/system.md.
Why is this needed?
Although GEMINI_SYSTEM_MD can be used to specify an explicit path, once a path is set, .gemini/system.md inside certain repositories will no longer take effect, which is inconvenient for our team.
This feature allows our team to keep a consistent global setup while still easily enabling repository-specific .gemini/system.md files when needed.
The behavior is intuitive and provides good flexibility.
Additional context
No response