[P14] Implement blast_radius tool

[four-bytes-robby]

Goal

Given a file, find everything that might break when you touch it — coupled files, shared authors, related modules.

Parameters

• file (string, required) — file path relative to repo root
• since (string, optional) — time window filter

Implementation

• Find implicitly coupled files (reuse coupling logic from [P14] Implement bus_factor tool #3)
• Find files with shared dominant authors (reuse ownership logic from [P14] Implement implicit_coupling tool #4)
• Find files in same directory that change frequently
• Return combined risk report: coupled files, shared-author files, co-changing neighbors
• Include risk scores per related file

Tests

• Test with known blast radius results
• Verify combined output from coupling + ownership data
• Test with file that has no coupling