[DOCS] Add an AGENTS.md (#9532)

We just describe a few basic things for now to help the agent figure out
how to use lit tests and pytests effectively.

Author: lezcano

.gitignore

@@ -94,3 +94,5 @@ docs/sg_execution_times.rst
 
 # macOS
 .DS_Store
+
+AGENTS.override.md

AGENTS.md

@@ -0,0 +1,10 @@
+# Working on Triton
+
+## Build and Testing Guidelines
+- Before running any tests, run `make` in the triton directory to rebuild triton.
+- For compiler changes, add tests in `python/test/` (pytest) or test (lit). Keep GPU-only tests in `python/test/unit/` or `python/test/gluon/`, name them `test_<feature>_<condition>`, and avoid creating new test files unless requested.
+- Run pytest with `-s --tb=short`. Run a single test with `pytest file.py::test_name`.
+- The build dir is given by `BUILD_DIR := $(shell cd python; $(PYTHON) -c 'from build_helpers import get_cmake_dir; print(get_cmake_dir())')`
+- Run lit from the build dir:  `cd BUILD_DIR; ninja triton-opt; lit -v test/<path>.mlir` (example: `lit -v test/TritonNvidiaGPU/tmem_layouts.mlir`).
+- Lit tests can be run locally (no GPU required).
+- Compiler crashes sometimes print an MLIR reproducer (external_resources / mlir_reproducer). Save the full MLIR + {-# ... #-} metadata to `/tmp/<file>.mlir`, then run `triton-opt /tmp/<file>.mlir --run-reproducer` to reproduce locally.