feat(inspector): improve session UI, skills dropdown, and visual polish (#179)

- Add delete button on ended sessions (visible on hover)
- Darken ended sessions with opacity and "ended" pill badge
- Sort ended sessions to bottom of list
- Add token usage pill in chat header
- Disable input when session ended
- Add Official Skills dropdown with SDK and Rivet presets
- Format session IDs shorter with full ID on hover
- Add arrow icon to "Configure persistence" link
- Add agent logo SVGs

frontend/packages/inspector/src/components/chat/messageUtils.tsx

@@ -1,5 +1,5 @@
 import type { TimelineEntry } from "./types";
-import { Settings, AlertTriangle, User } from "lucide-react";
+import { Settings, AlertTriangle } from "lucide-react";
 import type { ReactNode } from "react";
 
 export const getMessageClass = (entry: TimelineEntry) => {
@@ -11,7 +11,7 @@ export const getMessageClass = (entry: TimelineEntry) => {
 };
 
 export const getAvatarLabel = (messageClass: string): ReactNode => {
-  if (messageClass === "user") return <User size={14} />;
+  if (messageClass === "user") return null;
   if (messageClass === "tool") return "T";
   if (messageClass === "system") return <Settings size={14} />;
   if (messageClass === "error") return <AlertTriangle size={14} />;