feat: git blame ownership + bus factor metrics

[bntvllnt]

Summary

Add file/module ownership metrics derived from git blame, including bus factor (how many authors would need to leave before knowledge is lost).

Motivation

• CodeScene's biggest differentiator is team dynamics and bus factor analysis
• Ownership data helps AI agents know who to tag for review
• Bus factor identifies knowledge concentration risks
• git blame data is free — we already parse git log for churn

Proposed Metrics

Metric	Type	Source	Meaning
primaryOwner	string	git blame	Author with most lines
ownershipConcentration	0-1	git blame	How concentrated ownership is (1 = single author)
busFactor	1-N	git blame	Min authors covering 80% of lines
lastModifiedBy	string	git log	Most recent committer
activeContributors	number	git log (90d)	Authors active in last 90 days

Acceptance Criteria

• git blame parsed per file (batch, not per-line subprocess)
• Ownership metrics added to FileMetrics
• Bus factor computed per module
• Exposed in file_context and find_hotspots tools
• Tests with real git repos

Priority

Short-term — Bridges gap with CodeScene at minimal effort.