fix: Track the codegen content seed

The CODEGEN.md gitignore rule matches any depth, so the Metalsmith
source seed codegen/content/CODEGEN.md was never committed and CI
failed to generate with ENOENT on the missing directory. Force-add the
seed exactly as seamapi/javascript-http does; the ignore rule still
covers the generated CODEGEN.md at the repository root.

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

Author: claude

codegen/content/CODEGEN.md

@@ -0,0 +1 @@
+# Codegen