Add wg-mtu-auto initial implementation, documentation, and unit tests

- Added main.py: automatic WireGuard MTU calculation and PMTU probing
- Added test.py: unittests covering base, PMTU, and fallback scenarios
- Added Makefile: includes test target and install guidance
- Added README.md: usage, pkgmgr installation, and MIT license

Reference: https://chatgpt.com/share/68efc179-1a10-800f-9656-1e8731b40546

Makefile

@@ -0,0 +1,19 @@
+PY ?= python3
+
+.PHONY: test install help
+
+help:
+	@echo "Targets:"
+	@echo "  make test      - run unit tests"
+	@echo "  make install   - print installation guidance"
+	@echo "  make help      - this help"
+
+test:
+	$(PY) -m unittest -v test.py
+
+install:
+	@echo "Installation is provided via your package manager:"
+	@echo "  pkgmgr install automtu"
+	@echo ""
+	@echo "Alternatively, run the tool directly:"
+	@echo "  $(PY) main.py [--options]"