Update README.md badges to track 0.4.x branch

Changed CI and codecov badges to track the 0.4.x branch instead of
master, so the badges accurately reflect the status of this stable
release branch.

Assisted-by: Claude Code

Author: twaugh

README.md

@@ -1,7 +1,7 @@
 # Patchutils
 
-[![CI](https://github.com/twaugh/patchutils/actions/workflows/ci.yml/badge.svg)](https://github.com/twaugh/patchutils/actions/workflows/ci.yml)
-[![codecov](https://codecov.io/gh/twaugh/patchutils/branch/master/graph/badge.svg)](https://codecov.io/gh/twaugh/patchutils)
+[![CI](https://github.com/twaugh/patchutils/actions/workflows/ci.yml/badge.svg?branch=0.4.x)](https://github.com/twaugh/patchutils/actions/workflows/ci.yml?query=branch%3A0.4.x)
+[![codecov](https://codecov.io/gh/twaugh/patchutils/branch/0.4.x/graph/badge.svg)](https://codecov.io/gh/twaugh/patchutils/tree/0.4.x)
 
 A collection of tools that operate on patch files.