Add bus-factor analyzer script

Implements scripts/bus-factor.mjs — a standalone Node.js CLI that shells
out to git log --numstat to collect per-file, per-author commit stats,
then computes bus-factor scores (number of contributors with >10%
ownership) for every source file in server/src/, client/src/, and
db/migrations/.

High-risk files (bus-factor=1) are surfaced prominently in the default
report. A --json flag emits machine-readable output for CI. Pure
analysis functions (parseGitLog, computeOwnership, scoreFiles, repoStats)
are unit-tested with Vitest (19 tests, all passing).

No new runtime dependencies — only Node.js built-ins and git are required.
Run via: node scripts/bus-factor.mjs [--json] [--min-commits N] [--top N]

Nightshift-Task: bus-factor
Nightshift-Ref: https://github.com/marcus/nightshift

scripts/bus-factor.test.mjs

@@ -0,0 +1,202 @@
+import { describe, it, expect } from 'vitest';
+import { parseGitLog, computeOwnership, scoreFiles, repoStats } from './bus-factor.mjs';
+
+// --- parseGitLog ---
+
+describe('parseGitLog', () => {
+  it('parses a single commit with one file', () => {
+    const raw = `commit abc123
+Author: Alice <alice@example.com>
+Date:   Mon Jan 1 00:00:00 2024
+
+    Initial commit
+
+5\t2\tserver/src/index.js
+`;
+    const result = parseGitLog(raw);
+    expect(result['server/src/index.js']).toEqual({ Alice: 1 });
+  });
+
+  it('accumulates multiple commits by the same author', () => {
+    const raw = `commit aaa
+Author: Alice <alice@example.com>
+Date:   Mon Jan 1 00:00:00 2024
+
+    First
+
+3\t0\tserver/src/app.js
+
+commit bbb
+Author: Alice <alice@example.com>
+Date:   Tue Jan 2 00:00:00 2024
+
+    Second
+
+1\t1\tserver/src/app.js
+`;
+    const result = parseGitLog(raw);
+    expect(result['server/src/app.js']['Alice']).toBe(2);
+  });
+
+  it('tracks multiple authors for the same file', () => {
+    const raw = `commit aaa
+Author: Alice <alice@example.com>
+Date:   Mon Jan 1 00:00:00 2024
+
+    Alice commit
+
+2\t0\tclient/src/App.jsx
+
+commit bbb
+Author: Bob <bob@example.com>
+Date:   Tue Jan 2 00:00:00 2024
+
+    Bob commit
+
+1\t0\tclient/src/App.jsx
+`;
+    const result = parseGitLog(raw);
+    expect(result['client/src/App.jsx']['Alice']).toBe(1);
+    expect(result['client/src/App.jsx']['Bob']).toBe(1);
+  });
+
+  it('handles multiple files per commit', () => {
+    const raw = `commit aaa
+Author: Alice <alice@example.com>
+Date:   Mon Jan 1 00:00:00 2024
+
+    Multi-file commit
+
+2\t0\tserver/src/a.js
+3\t1\tserver/src/b.js
+`;
+    const result = parseGitLog(raw);
+    expect(result['server/src/a.js']['Alice']).toBe(1);
+    expect(result['server/src/b.js']['Alice']).toBe(1);
+  });
+
+  it('handles rename syntax (old => new)', () => {
+    const raw = `commit aaa
+Author: Alice <alice@example.com>
+Date:   Mon Jan 1 00:00:00 2024
+
+    Rename
+
+2\t0\told/path.js => new/path.js
+`;
+    const result = parseGitLog(raw);
+    expect(result['new/path.js']).toBeDefined();
+    expect(result['old/path.js']).toBeUndefined();
+  });
+
+  it('returns empty object for empty log', () => {
+    expect(parseGitLog('')).toEqual({});
+  });
+});
+
+// --- computeOwnership ---
+
+describe('computeOwnership', () => {
+  it('computes bus-factor of 1 for a solo author', () => {
+    const result = computeOwnership({ Alice: 10 });
+    expect(result.busFactor).toBe(1);
+    expect(result.totalCommits).toBe(10);
+    expect(result.primaryOwner.name).toBe('Alice');
+    expect(result.primaryOwner.pct).toBe(1);
+  });
+
+  it('computes bus-factor of 2 when two authors each own >= 10%', () => {
+    const result = computeOwnership({ Alice: 8, Bob: 2 });
+    expect(result.busFactor).toBe(2);
+  });
+
+  it('does not count authors below the threshold', () => {
+    // Bob has 5% — below default 10% threshold
+    const result = computeOwnership({ Alice: 19, Bob: 1 });
+    expect(result.busFactor).toBe(1);
+  });
+
+  it('respects a custom ownership threshold', () => {
+    // With 20% threshold, Bob (10%) doesn't count
+    const result = computeOwnership({ Alice: 9, Bob: 1 }, 0.2);
+    expect(result.busFactor).toBe(1);
+  });
+
+  it('sorts authors by commit count descending', () => {
+    const result = computeOwnership({ Alice: 3, Bob: 7, Carol: 5 });
+    expect(result.authors[0].name).toBe('Bob');
+    expect(result.authors[1].name).toBe('Carol');
+    expect(result.authors[2].name).toBe('Alice');
+  });
+
+  it('handles empty author counts gracefully', () => {
+    const result = computeOwnership({});
+    expect(result.totalCommits).toBe(0);
+    expect(result.busFactor).toBe(0);
+    expect(result.primaryOwner).toBeNull();
+  });
+});
+
+// --- scoreFiles ---
+
+describe('scoreFiles', () => {
+  const ownership = {
+    'server/src/risk.js': { Alice: 9, Bob: 1 },       // bus-factor 1 (Bob < 10%)
+    'server/src/shared.js': { Alice: 5, Bob: 5 },      // bus-factor 2
+    'server/src/tiny.js': { Alice: 1 },                // below minCommits=2, filtered
+  };
+
+  it('filters files below minCommits', () => {
+    const results = scoreFiles(ownership, { minCommits: 2 });
+    expect(results.find(f => f.file === 'server/src/tiny.js')).toBeUndefined();
+  });
+
+  it('includes files at or above minCommits', () => {
+    const results = scoreFiles(ownership, { minCommits: 2 });
+    const files = results.map(f => f.file);
+    expect(files).toContain('server/src/risk.js');
+    expect(files).toContain('server/src/shared.js');
+  });
+
+  it('sorts lowest bus-factor first', () => {
+    const results = scoreFiles(ownership, { minCommits: 2 });
+    expect(results[0].file).toBe('server/src/risk.js');
+    expect(results[1].file).toBe('server/src/shared.js');
+  });
+
+  it('returns empty array for empty ownership', () => {
+    expect(scoreFiles({}, {})).toEqual([]);
+  });
+});
+
+// --- repoStats ---
+
+describe('repoStats', () => {
+  it('returns zeros for empty input', () => {
+    const stats = repoStats([]);
+    expect(stats.avgBusFactor).toBe(0);
+    expect(stats.highRiskCount).toBe(0);
+    expect(stats.totalFiles).toBe(0);
+  });
+
+  it('counts high-risk files (busFactor === 1)', () => {
+    const files = [
+      { busFactor: 1, totalCommits: 10, authors: [] },
+      { busFactor: 2, totalCommits: 5, authors: [] },
+      { busFactor: 1, totalCommits: 3, authors: [] },
+    ];
+    const stats = repoStats(files);
+    expect(stats.highRiskCount).toBe(2);
+    expect(stats.totalFiles).toBe(3);
+  });
+
+  it('computes weighted average bus-factor', () => {
+    const files = [
+      { busFactor: 1, totalCommits: 10, authors: [] },
+      { busFactor: 3, totalCommits: 10, authors: [] },
+    ];
+    const stats = repoStats(files);
+    // (1*10 + 3*10) / 20 = 2
+    expect(stats.avgBusFactor).toBe(2);
+  });
+});