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/public/logos/pi.svg

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 800 800">
+  <!-- P shape: outer boundary clockwise, inner hole counter-clockwise -->
+  <path fill="#fff" fill-rule="evenodd" d="
+    M165.29 165.29
+    H517.36
+    V400
+    H400
+    V517.36
+    H282.65
+    V634.72
+    H165.29
+    Z
+    M282.65 282.65
+    V400
+    H400
+    V282.65
+    Z
+  "/>
+  <!-- i dot -->
+  <path fill="#fff" d="M517.36 400 H634.72 V634.72 H517.36 Z"/>
+</svg>