loop: add diff stats from sketch-base to HEAD in /state endpoint

Add lines added/removed statistics computed from sketch-base to current HEAD,
displayed in webui Diff mode button for quick change overview.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s3f10ecf39df6b581k
diff --git a/webui/src/fixtures/dummy.ts b/webui/src/fixtures/dummy.ts
index 87b73bd..2d8d91f 100644
--- a/webui/src/fixtures/dummy.ts
+++ b/webui/src/fixtures/dummy.ts
@@ -376,4 +376,6 @@
   in_container: true,
   first_message_index: 0,
   agent_state: "WaitingForUserInput",
+  diff_lines_added: 42,
+  diff_lines_removed: 7,
 };