Add an AGENTS.md

Author: pavpanchekha

AGENTS.md

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+# Organization
+
+- The MPFR bindings live in `mpfr.rkt`; the base interval definition
+  is in `ops/core.rkt`; the interval arithmetic functions are in
+  `ops/`; the real evaluator is in `eval/`; the REPL is in `repl.rkt`.
+  The tests are in `test.rkt`; the nightly is `time.rkt`. The nightly
+  also has various helper functions in `infra/`
+- When editing an interval arithmetic function, always run the tests.
+  Run tests with `racket -y test.rkt add` (for the right function).