all: add GEMINI.md

Add an initial version of the GEMINI.md file for use by coding agents.

Author: a-nogikh

GEMINI.md

@@ -0,0 +1,21 @@
+# Project: syzkaller / syzbot
+
+## General Instructions
+
+Prefer to use the `syz-env` Docker container to build and run tests. There's a `./tools/syz-env` script that will start a container and run the given command inside it.
+
+Some reference commands:
+* Linter checks: `CI=true ./tools/syz-env make lint`
+* Run a test: `CI=true ./tools/syz-env go test ./package -run TestName`
+
+Note the `CI=true` part - otherwise the commands may not run in your environment.
+
+Err on the side of adding tests for your changes. For some packages (e.g. `./dashboard/app` or `./prog`), we strive to have a very high test coverage. When adding a test, carefully look at the other tests in the same package to understand the style and structure.
+
+When running tests (especially in `./prog` and `./pkg/csource`) prefer to run individual tests you have affected, otherwise it may take a lot of time. Some packages also offer a `-short` flag to run a lighter version of tests.
+
+It may be necessary to first run `CI=true ./tools/syz-env make descriptions` to pre-build descriptions for the `sys/*` targets. It may be necessary for all tests that eventually use `prog.Target` or `targets.Target`.
+
+Refer to the documentation in `docs/` regarding syzlang syntax and instructions for various syzkaller tools.
+
+When you add new .go files, make sure to add the copyright header to them (use other .go files for reference and update the year to the current one).