fix: take snapshot before stream starts to fix diff tracking for some providers

Some providers emit the start-step event after tool execution has already
begun, causing step-start and step-finish to have identical snapshots.
This results in empty diffs and missing Modified Files in the UI.

By taking the initial snapshot before the stream starts, we ensure we
always capture the true 'before' state regardless of provider timing.

Author: processtrader

packages/opencode/src/session/processor.ts

@@ -40,6 +40,10 @@ export namespace SessionProcessor {
       },
       async process(fn: () => StreamTextResult<Record<string, AITool>, never>) {
         log.info("process")
+        // Take initial snapshot BEFORE stream starts to ensure we capture
+        // the "before" state regardless of when start-step event fires
+        // (some providers emit start-step after tool execution begins)
+        const initialSnapshot = await Snapshot.track()
         while (true) {
           try {
             let currentText: MessageV2.TextPart | undefined
@@ -229,7 +233,9 @@ export namespace SessionProcessor {
                   throw value.error
 
                 case "start-step":
-                  snapshot = await Snapshot.track()
+                  // Use initialSnapshot taken before stream started to ensure
+                  // we have the true "before" state (some providers emit start-step late)
+                  snapshot = initialSnapshot
                   await Session.updatePart({
                     id: Identifier.ascending("part"),
                     messageID: input.assistantMessage.id,