Add AGENTS guidelines

AGENTS.md

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+# AGENTS Guidelines for Sortana
+
+This file provides guidelines for codex agents contributing to the Sortana project. It describes the repository structure, the expected coding conventions, and testing commands. If a section is not applicable yet, you may leave it blank or provide placeholders.
+
+## Repository Overview
+
+- `background.js`: Handles startup tasks and message passing between the extension and experiment APIs.
+- `experiment/`: Contains the privileged API scripts used by Thunderbird.
+- `modules/`: Holds reusable JavaScript modules for the extension.
+- `content/`: Scripts for modifying Thunderbird windows (e.g., the filter editor).
+- `options/`: The options page HTML and JavaScript.
+- `resources/`: Images and other static files.
+- `prompt_templates/`: Prompt template files for the AI service.
+- `build-xpi.ps1`: PowerShell script to package the extension.
+
+## Coding Style
+
+- Use **modern JavaScript** (ES6 or later). Prefer `const` and `let` over `var`.
+- Keep functions small and focused. Aim for clear naming and concise comments when necessary.
+- Use template literals for string interpolation.
+- Avoid trailing whitespace and ensure files end with a newline.
+
+## Commit Guidelines
+
+- Group related changes together. Each commit should have a clear purpose.
+- Use descriptive commit messages in the imperative mood (e.g., "Add filter editor patch").
+- Run `git status` before committing to verify only intended files are staged.
+
+## Testing
+
+There are currently no automated tests for this project. If you add tests in the future, specify the commands to run them here. For now, verify the extension manually in Thunderbird.
+
+## Documentation
+
+Additional documentation might exist outside this repository. Replace the placeholders below with the correct URLs if available.
+
+- Development guide: <URL to development docs>
+- Issue tracker: <URL to issue tracker>
+- Extension homepage: <URL to extension site>
+