Add initial AGENTS.md file (#1352)

We can fill it in more as we go


<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Documentation**
* Added testing guidance for the NullAway module, including instructions
for running and filtering tests with build tools.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: msridhar

AGENTS.md

@@ -0,0 +1,10 @@
+# Testing
+
+To run only the tests for the main NullAway module, run `./gradlew :nullaway:test`. Run _only_ these tests unless you
+are specifically asked to run a test in a different module. If you want to run a single
+test class or method within that module, you can use the `--tests` flag. For example, to run all tests in the
+`com.uber.nullaway.NullAwayTest` class, you would run:
+
+```bash
+./gradlew :nullaway:test --tests "com.uber.nullaway.NullAwayTest"
+```

CLAUDE.md

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+AGENTS.md