Update AGENTS guidelines (#3556)

## Summary
- clarify Codex contribution instructions
- remove `test.sh` reference and require `./gradlew build`
- add Developer Guide, AI note and translation policy

## Testing
- `./gradlew spotlessApply`
- `./gradlew build`

AGENTS.md

@@ -0,0 +1,24 @@
+# Codex Contribution Guidelines for Stirling-PDF
+
+This file provides high-level instructions for Codex when modifying any files within this repository. Follow these rules to ensure changes remain consistent with the existing project structure.
+
+## 1. Code Style and Formatting
+- Respect the `.editorconfig` settings located in the repository root. Java files use 4 spaces; HTML, JS, and Python generally use 2 spaces. Lines should end with `LF`.
+- Format Java code with `./gradlew spotlessApply` before committing.
+- Review `DeveloperGuide.md` for project structure and design details before making significant changes.
+
+## 2. Testing
+- Run `./gradlew build` before committing changes to ensure the project compiles.
+- If the build cannot complete due to environment restrictions, DO NOT COMMIT THE CHANGE
+
+## 3. Commits
+- Keep commits focused. Group related changes together and provide concise commit messages.
+- Ensure the working tree is clean (`git status`) before concluding your work.
+
+## 4. Pull Requests
+- Summarize what was changed and why. Include build results from `./gradlew build` in the PR description.
+- Note that the code was generated with the assistance of AI.
+
+## 5. Translations
+- Only modify `messages_en_GB.properties` when adding or updating translations.
+