Add AGENTS.md for Cursor Cloud development environment setup

AGENTS.md

@@ -0,0 +1,28 @@
+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+This is a **pure Go test-utility library** with zero external dependencies beyond the Go toolchain and `git` CLI. There are no services, databases, or containers to start.
+
+### Prerequisites
+
+- Go 1.22+ and `git` must be on `PATH` (both are pre-installed in the Cloud Agent VM).
+
+### Key commands
+
+All commands are run from the repo root (`/workspace`):
+
+| Task | Command |
+|------|---------|
+| Lint (vet) | `go vet ./...` |
+| Format check | `gofmt -l *.go` |
+| Format fix | `gofmt -w *.go` |
+| Run tests | `go test ./...` |
+| Verbose tests | `go test -v ./...` |
+
+See `DEVELOPER_GUIDE.md` for full contributor workflow and PR checklist.
+
+### Caveats
+
+- The full test suite (`go test ./...`) takes ~90 seconds due to `TestCreateMultiBranchScenario`, `TestSnapshotAndRestore`, and `TestCreateTestRepo_Dirty` which create many git commits. Use `go test -short ./...` for faster iteration if the tests under change don't need the large-repo scenarios.
+- `gofmt` is the only formatting tool used; there is no `golangci-lint` config.