fix: codesign gt binary after install on macOS

The build target was signing the binary, but install just copied
it without re-signing. On macOS, copying can invalidate signatures.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Makefile

@@ -24,6 +24,9 @@ endif
 
 install: build
 	cp $(BUILD_DIR)/$(BINARY) ~/.local/bin/$(BINARY)
+ifeq ($(shell uname),Darwin)
+	@codesign -s - -f ~/.local/bin/$(BINARY) 2>/dev/null || true
+endif
 
 clean:
 	rm -f $(BUILD_DIR)/$(BINARY)