Create high scores and agent stats tab

[areibman]

Add a new "High Scores" tab to display key performance metrics for human and agent contributors, including a dedicated section for agent PR statistics.

This PR provides engineering managers with valuable insights into contributor activity and agent performance, fulfilling the request for specific stats like merged PRs, time to merge, PR size, and per-agent "PRs Raised : Merged" ratios.

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Fixes #170

[cursor[bot]]

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