fix(vitest): cap worker parallelism to prevent process storms (#500)

Vitest v3 defaults to `pool: "forks"` and scales worker processes with
CPU count. This repo's tests spawn many Node processes (CLI invocations,
temp FS operations), which can cause runaway CPU/memory usage when
combined with high parallelism.

Changes:
- Explicitly set pool to 'forks' (tests rely on process isolation)
- Add resolveMaxWorkers() to cap workers at min(4, availableCPUs)
- Allow VITEST_MAX_WORKERS env override for CI/automation tuning

vitest.config.ts

@@ -1,12 +1,33 @@
 import { defineConfig } from 'vitest/config';
+import os from 'node:os';
+
+function resolveMaxWorkers(): number | undefined {
+  // Allow callers (CI/agents) to override without editing config.
+  const raw = process.env.VITEST_MAX_WORKERS;
+  if (raw) {
+    const parsed = Number(raw);
+    if (Number.isFinite(parsed) && parsed > 0) {
+      return parsed;
+    }
+  }
+
+  // Vitest v3 defaults to `pool: "forks"` and scales worker processes with CPU.
+  // This repo's tests can spawn many Node processes (CLI invocations, temp FS),
+  // so cap parallelism to avoid runaway CPU/memory usage in automation.
+  const cpuCount = typeof os.availableParallelism === 'function'
+    ? os.availableParallelism()
+    : os.cpus().length;
+  return Math.min(4, Math.max(1, cpuCount));
+}
 
 export default defineConfig({
   test: {
     globals: true,
     environment: 'node',
     globalSetup: './vitest.setup.ts',
-    // Keep default pool settings; some tests rely on process.chdir,
-    // which is not supported in worker threads
+    // Tests rely on per-file process isolation (e.g., `process.cwd()` assumptions).
+    pool: 'forks',
+    maxWorkers: resolveMaxWorkers(),
     include: ['test/**/*.test.ts'],
     coverage: {
       reporter: ['text', 'json', 'html'],