docs: add AGENTS.md with commit conventions

AGENTS.md

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+# Agent instructions
+
+## Commits
+
+Use [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/) for commit subjects:
+`<type>(<optional scope>): <description>`, e.g. `feat(membership): add Apple Pay` or
+`chore(deps): upgrade to Expo SDK 58`. Common types are `feat`, `fix`, `refactor`, `test`, `docs`,
+`style`, `perf`, `build`, `ci` and `chore`. Write the description in the imperative mood and in
+lowercase.