Diffs: restore system prompt guidance (#36904)

Merged via squash.

Prepared head SHA: 1b3be3c879
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

extensions/diffs/index.ts

@@ -7,6 +7,7 @@ import {
   resolveDiffsPluginSecurity,
 } from "./src/config.js";
 import { createDiffsHttpHandler } from "./src/http.js";
+import { DIFFS_AGENT_GUIDANCE } from "./src/prompt-guidance.js";
 import { DiffArtifactStore } from "./src/store.js";
 import { createDiffsTool } from "./src/tool.js";
 
@@ -34,6 +35,9 @@ const plugin = {
         allowRemoteViewer: security.allowRemoteViewer,
       }),
     });
+    api.on("before_prompt_build", async () => ({
+      prependSystemContext: DIFFS_AGENT_GUIDANCE,
+    }));
   },
 };