Add AGENTS.md files for typespec models

[amansinghoriginal]

Description

Add AGENTS.md files for typespec models to provide relevant context to coding agents.

Works for:

• Gemini CLI
• Codex (OpenAI)
• Github Copilot Coding Agent
• Web-based Copilot Agent (SWE agent)

Does not work for:

• Github Copilot inline completions (VS Code / Visual Studio)
• Claude Code (Mitigated by creating a symlink called CLAUDE.md for each AGENT.md file)

Type of change

• This pull request fixes a bug in Drasi and has an approved issue (issue link required).
• This pull request adds or changes features of Drasi and has an approved issue (issue link required).
• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Drasi (issue link optional).