[P14] Implement implicit_coupling tool

[four-bytes-robby]

Goal

Detect files that always change together in the same commit — hidden dependencies invisible in code.

Parameters

• threshold (number, optional, default: 0.8) — minimum co-commit rate
• since (string, optional) — time window filter

Implementation

• Parse all commits, extract file lists per commit
• Build co-occurrence matrix for file pairs
• Compute coupling strength = co-commits / max(individual commits)
• Return pairs above threshold, ranked by strength
• Cap at reasonable limit (e.g., top 50 pairs)

Tests

• Test with known coupled file pairs
• Verify threshold filtering
• Test empty repo edge case