Add AGENTS.md with build/test instructions

Author: asgerf

unified/AGENTS.md

@@ -0,0 +1,15 @@
+# Agent instructions
+
+This is a CodeQL extractor based on tree-sitter.
+
+## Building
+To build the extractor, run `scripts/create-extractor-pack.sh`
+
+## Testing
+- If you changed the extractor code, always rebuild it before running tests.
+
+- To run all tests, run `codeql test run --search-path extractor-pack ql/test`
+
+- Do not edit `.expected` files manually. To update the expected output, pass `--learn` to the `codeql test run` command.
+
+- To run a specific test, pass the specific directory to the `codeql test run` command instead of `ql/test`.