fix: Move codegen gitignore rules to conventional positions

CODEGEN.md moves to the top of the file. The codegen/lib/ negation
moves next to the lib/ pattern it negates, since a gitignore negation
only takes effect after the pattern it overrides.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019iX1vWXfXKzUyRJTVm8D7s

Author: claude

.gitignore

@@ -5,6 +5,9 @@
 # For more details, visit the project page:
 # https://github.com/github/gitignore
 
+# Codegen
+CODEGEN.md
+
 # Temporary development files
 tmp
 
@@ -28,6 +31,8 @@ downloads/
 eggs/
 .eggs/
 lib/
+# Negations must follow the pattern they negate, so this cannot move to the top
+!codegen/lib/
 lib64/
 parts/
 sdist/
@@ -325,7 +330,3 @@ dist
 # yalc
 .yalc/
 yalc.lock
-
-# Codegen
-!codegen/lib/
-CODEGEN.md