feat(agent): add git diff viewer and improve permission handling

- Add git diff viewer screen accessible via agent dropdown menu
- Implement git_diff_request/response WebSocket messages
- Add dropdown menu to agent screen header with "View Changes" option
- Clear pending permissions when starting new agent turn to prevent conflicts
- Treat agent refusals as completed rather than failed state
- Add test coverage for permission handling edge cases
- Configure vitest to run in single-threaded mode for stability
- Remove redundant wrapper in agent stream view

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

packages/app/src/components/agent-stream-view.tsx

@@ -267,28 +267,24 @@ export function AgentStreamView({
         onMomentumScrollEnd={handleScrollEnd}
         scrollEventThrottle={16}
         ListEmptyComponent={
-          <View style={stylesheet.invertedWrapper}>
-            <View style={stylesheet.emptyState}>
-              <Text style={stylesheet.emptyStateText}>
-                Start chatting with this agent...
-              </Text>
-            </View>
+          <View style={stylesheet.emptyState}>
+            <Text style={stylesheet.emptyStateText}>
+              Start chatting with this agent...
+            </Text>
           </View>
         }
         ListHeaderComponent={
-          <View style={stylesheet.invertedWrapper}>
-            {pendingPermissionItems.length > 0 ? (
-              <View style={stylesheet.permissionsContainer}>
-                {pendingPermissionItems.map((permission) => (
-                  <PermissionRequestCard
-                    key={permission.requestId}
-                    permission={permission}
-                    onResponse={onPermissionResponse}
-                  />
-                ))}
-              </View>
-            ) : null}
-          </View>
+          pendingPermissionItems.length > 0 ? (
+            <View style={stylesheet.permissionsContainer}>
+              {pendingPermissionItems.map((permission) => (
+                <PermissionRequestCard
+                  key={permission.requestId}
+                  permission={permission}
+                  onResponse={onPermissionResponse}
+                />
+              ))}
+            </View>
+          ) : null
         }
         extraData={pendingPermissionItems.length}
         maintainVisibleContentPosition={{