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

Author: NicholasKissel