add codeowners (#122)

Author: lnhsingh

.github/CODEOWNERS

@@ -0,0 +1,19 @@
+# Owner of the CODEOWNERS
+/.github/CODEOWNERS @lnhsingh
+
+# These owners will be the default owners for everything in
+# the repo. Unless a later match takes precedence, they will be requested for review when someone opens a pull request.
+*       @lnhsingh
+
+# Any file in the `/src/langgraph-platform`
+# and any of its subdirectories.
+/src/langgraph-platform/ @katmayb @lnhsingh
+
+# Any file in the `/src/langsmith`
+# and any of its subdirectories.
+/src/langsmith/ @katmayb @lnhsingh
+
+# Any file in the `/src/oss`
+# and any of its subdirectories.
+/src/oss/ @beklapko @lnhsingh
+