docs: add observe-and-operate capability to overview

[codeaholicguy]

Verified finding

• Doc claim (0-what-is-ai-devkit.md): the overview's capabilities and "How It Works" never mention status, capacity, or the daemon.
• Reality: all three shipped (status ~0.59, capacity 0.63–0.68, devkitd 0.69) — packages/cli/src/commands/{status,capacity,daemon}.ts.

What changed

• New "Observe And Operate" section covering status, capacity (six providers), and the auto-starting devkitd daemon
• "How It Works" Operate step extended to include status/capacity

Note: the new section links to the Runtime & Capacity page proposed in #354; if that lands later this link resolves once it merges.

Validation

• npm run build in web/: pass
• npm run lint at root: pass

Generated with Devin

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technical-writer review pass: ran; no issues flagged.