Add AGENTS.md with project guidelines

Document GNOME commit style, code principles, testing, and security
practices for contributors.

Changes:
- Include commit guidelines
- Code style notes
- Testing and security reminders

AGENTS.md

@@ -0,0 +1,17 @@
+# Project Guidelines
+
+## Commits
+Follow GNOME style: Summary <72 chars, imperative, no period. Body with bullet points for changes.
+
+## Code
+- Classes for modularity, private attrs/methods (_prefix).
+- Avoid redundancy (e.g., single strip() calls).
+- Encapsulation over public access.
+
+## Testing
+- Dry-run first, check logs/counts vs. Thunderbird.
+- Verify no duplicates or errors.
+
+## Security
+- Never commit .env; use .env.example.
+- Private credentials, no hard-coded URLs.