Add install/uninstall safety check via --signature flag

Automated-By: Claude Sonnet 4.6

Makefile

@@ -1,13 +1,27 @@
-PREFIX ?= $(HOME)/.local/bin
+PREFIX  ?= $(HOME)/.local/bin
+TARGET  := $(PREFIX)/CodeMapper
+SIG     := CODEMAPPER-SIGNATURE-izackp
 
-.PHONY: build install uninstall
+.PHONY: build install uninstall _check_existing
 
 build:
 	swift build -c release
 
-install: build
-	install -d $(PREFIX)
-	install .build/release/CodeMapper $(PREFIX)/CodeMapper
+_check_existing:
+	@if [ -f "$(TARGET)" ]; then \
+		GOT=$$("$(TARGET)" --signature 2>/dev/null || true); \
+		if [ "$$GOT" != "$(SIG)" ]; then \
+			if [ "$(FORCE)" != "1" ]; then \
+				echo "ERROR: $(TARGET) exists but is not this CodeMapper build."; \
+				echo "       Pass FORCE=1 to override."; \
+				exit 1; \
+			fi; \
+		fi \
+	fi
 
-uninstall:
-	rm -f $(PREFIX)/CodeMapper
+install: build _check_existing
+	install -d $(PREFIX)
+	install .build/release/CodeMapper $(TARGET)
+
+uninstall: _check_existing
+	rm -f $(TARGET)