feat(protocol): add DiffPreview type and wire into can_use_tool emission (#1151)

Co-authored-by: Letta <noreply@letta.com>

src/headless.ts

@@ -73,6 +73,7 @@ import {
   validateRegistryHandleOrThrow,
 } from "./cli/startupFlagValidation";
 import { SYSTEM_REMINDER_CLOSE, SYSTEM_REMINDER_OPEN } from "./constants";
+import { computeDiffPreviews } from "./helpers/diffPreview";
 import {
   mergeQueuedTurnInput,
   type QueuedTurnInput,
@@ -2548,6 +2549,9 @@ async function runBidirectionalMode(
   }> {
     const requestId = `perm-${toolCallId}`;
 
+    // Compute diff previews for file-modifying tools
+    const diffs = await computeDiffPreviews(toolName, toolInput);
+
     // Build can_use_tool control request (Claude SDK format)
     const canUseToolRequest: CanUseToolControlRequest = {
       subtype: "can_use_tool",
@@ -2556,6 +2560,7 @@ async function runBidirectionalMode(
       tool_call_id: toolCallId, // Letta-specific
       permission_suggestions: [], // TODO: not implemented
       blocked_path: null, // TODO: not implemented
+      ...(diffs.length > 0 ? { diffs } : {}),
     };
 
     const controlRequest: ControlRequest = {