Move AI advice to AGENTS.md and add symlink from CLAUDE.md (#5914)

# Description of Changes
Inspired by https://github.com/pydantic/pydantic-ai/pull/4169, this PR
moves our `CLAUDE.md` advice to the more generic `AGENTS.md` file (which
works on Codex, Gemini, etc). It also adds a symlink from `CLAUDE.md` to
`AGENTS.md`, which Claude follows properly, so all AIs should get the
same advice and we only need to keep one file up-to-date.

devGuide/AGENTS.md

@@ -1,24 +0,0 @@
-# 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.
-