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/vitest.config.mjs

@@ -0,0 +1,8 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+  },
+});