Add Alternative Agents page screenshot

PR preview asset. Captures the /alternative-agents route after the ACP benchmark data restoration (openhands-index-results #832-#843): all seven harness x model rows render complete, the scatter plot shows the cost-vs-score frontier populated, and the 'Show incomplete entries' toggle reports zero by default.

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+docs/screenshots/alternative-agents.png filter=lfs diff=lfs merge=lfs -text