[P14] Implement bus_factor tool

[four-bytes-robby]

Goal

Calculate bus factor per directory — identifies modules that would be orphaned if key contributors left.

Parameters

• since (string, optional) — time window filter

Implementation

• Aggregate git blame and commit data per directory
• Compute ownership concentration: what % of lines/changes does the top author own per directory
• Flag directories where one author owns >70%
• Return per-directory bus factor score and ownership breakdown

Tests

• Test with mock git blame/log output
• Verify bus factor = 1 detection
• Test directory grouping logic