Support gemini-specific (or agent-specific) instructions

[staktrace]

We have a repo (not using ai-rules) where the GEMINI.md file starts off like this:

# Gemini CLI-Specific Instructions

> **Note:** This file works alongside `AGENTS.md` (generic AI agent instructions). AGENTS.md contains the core Task Master commands and workflows for all AI agents. This file contains only Gemini CLI-specific features and integrations.

Does ai-rules support generating this sort of thing from a single source of truth?

[staktrace]

Fixing the issue in #25 would solve this problem for me. Also if symlink_mode: true separately symlinked AGENTS.md and GEMINI.md and CLAUDE.md and goosehints and so on that would also solve my problem.