Support referencing to other context files in Agent.md

[nedn]

Both Claude.md [1] and GEMINI.md [2] support referencing other context files through @.

It would be great if agent.md standard can support the same syntax for linking other context files.

[1] https://docs.claude.com/en/docs/claude-code/common-workflows#reference-files-and-directories

[2] https://google-gemini.github.io/gemini-cli/docs/core/memport.html