docs(readme): lead See-it with three premier screenshots

[umi-appcoder]

What

Makes three fresh screenshots the premier examples at the top of the See it section, full-width with captions:

1. Desktop home — dark theme, centered "What are we building" composer with the Mission Control strip (docs/screenshots/desktop-home-mission-strip.png)
2. Mission Control board — every agent across builds, chats and sub-agents in one queue (docs/screenshots/mission-control-board.png)
3. Hypervisor inline media — the chat capturing a screenshot and rendering it right in the thread (docs/screenshots/hypervisor-inline-media.png)

Incorporating the existing screenshots

• The previous gallery (Fullstack Python / Go split-panes, agent + app side by side, build sessions) is kept intact under a new More surfaces subheading.
• The old low-res Mission Control shot (pre three-column board) is superseded by the new board screenshot.
• The dashboard section swaps its stale pre-redesign desktop image (desktop-prompt-composer.png) for the already-committed desktop-redesign-light.png, so the README shows the current design in both dark (hero examples) and light (dashboard section).
• The Desktop bullet is updated to describe the current layout, and a Mission Control bullet is added.

No code changes — README + three PNGs only.

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