Build dmr.exe on Windows in build-dmr target

The build-dmr Makefile target always built a binary named "dmr",
which is unusable on Windows because the OS requires a .exe suffix
to execute it directly. Detect Windows via the built-in $(OS)
variable and use a DMR_EXE variable for the output filename, so the
target produces dmr.exe on Windows and dmr everywhere else. This
avoids shelling out to "go env GOEXE" on every invocation, which adds
overhead and can fail if go is not on the PATH. Reuse DMR_EXE in the
clean target instead of hardcoding both dmr and dmr.exe so cleanup
stays in sync with the build output. Update the README to match.

Author: ericcurtin

Makefile

@@ -65,11 +65,17 @@ DMR_LDFLAGS := -s -w \
 	-X main.Version=$(DMR_VERSION) \
 	-X github.com/docker/model-runner/cmd/cli/desktop.Version=$(DMR_VERSION)
 
+# Add .exe on windows, nothing elsewhere
+DMR_EXE := dmr
+ifeq ($(OS),Windows_NT)
+DMR_EXE := dmr.exe
+endif
+
 # build-dmr builds a native dmr binary. dmr has no cgo dependencies, so
 # CGO_ENABLED=0 keeps the binary statically linked and trivial to
 # cross-compile (see build-dmr-cross).
 build-dmr:
-	CGO_ENABLED=0 go build -ldflags="$(DMR_LDFLAGS)" -o dmr ./cmd/dmr
+	CGO_ENABLED=0 go build -ldflags="$(DMR_LDFLAGS)" -o $(DMR_EXE) ./cmd/dmr
 
 # build-dmr-cross builds standalone dmr binaries for every platform we
 # publish packages for (see packaging/), into dist/dmr/<os>-<arch>/dmr.
@@ -108,7 +114,7 @@ run: build
 # Clean build artifacts
 clean:
 	rm -f $(APP_NAME)
-	rm -f dmr
+	rm -f $(DMR_EXE)
 	rm -f model-runner.sock
 
 # Run tests

cmd/dmr/README.md

@@ -37,7 +37,7 @@ dmr rm ai/gemma3       # remove a local model
 ## Building
 
 ```
-make build-dmr         # native build, output ./dmr
+make build-dmr         # native build, output ./dmr (or dmr.exe on Windows)
 make build-dmr-cross   # cross-compile every published target into dist/dmr/<os>-<arch>/
 ```