Implement the Contributor Freshness time-decay algorithm

[SHAURYASANYAL3]

🛑 The Problem

A contributor who made 1,000 commits 5 years ago is far less relevant to the current 'Resilience' of a project than a contributor who made 50 commits yesterday. If we don't account for time, our Health Scores will falsely report abandoned projects as highly resilient.

💡 The Solution

We must apply an exponential time-decay formula to a contributor's weight.

🛠️ Implementation Details

1. Implement the mathematical formula: Weight = InitialValue * Math.exp(-lambda * timeElapsed).
2. Define the half-life (e.g., 180 days). This means a commit made 6 months ago is worth exactly half of a commit made today.
3. Apply this multiplier to the raw commit counts before passing them into the Bus Factor and Maintenance algorithms.

✅ Acceptance Criteria

• Older commits mathematically contribute less to the final score.
• The half-life is configurable via an environment variable or constant.
• Comprehensive unit tests prove the decay curve behaves correctly.

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