chore: add CODEOWNERS (#1)

All four maintainers are owners of every path. Required by the branch
protection rule about to land on main: at least one codeowner approval
unlocks merge.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: n-papaioannou

.github/CODEOWNERS

@@ -0,0 +1,4 @@
+# Code owners for ifixai-ai/diagnostic.
+# Listed users are auto-requested for review on every PR and at least one
+# of them must approve before main can be updated.
+* @n-papaioannou @Sebabaian @dimneo @stefyi-4355