perf(test): parallelize harder — unlock worker count, split build-output, bump workspace concurrency

- All 8 vitest configs now default maxWorkers to \`cpus().length - 1\`
  instead of the arbitrary \`Math.min(4, …)\` cap. Respects an explicit
  VITEST_MAX_WORKERS override for constrained environments (CI, laptops
  on battery). CLI keeps \`fileParallelism: false\` — audit found real
  shared state (process.chdir in agent-import, dist/ races in build-exe
  suites) that needs refactoring before we can flip it.
- packages/dashboard: move the mobile build-output smoke test (which
  invokes \`pnpm build:client\` via execSync, ~3s per run) into a
  dedicated \`test:build\` script so \`pnpm test\` isn't gated by it. A
  matching root script keeps CI wiring simple.
- Root: bump --workspace-concurrency from 2 → 4 so core/engine/cli/
  desktop can pipeline against dashboard's tail.
- Drop the now-redundant VITEST_MAX_WORKERS=4 prefix from the root
  scripts; the per-package configs pick up cpu count themselves.

Dashboard test suite: 182s → 38s (5x) on a 10-core machine. Full
workspace run: ~3m15 → 2m29. All tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/tui/vitest.config.ts

@@ -1,9 +1,10 @@
 import { defineConfig } from "vitest/config";
 import { fileURLToPath } from "node:url";
+import { cpus } from "node:os";
 
-const defaultMaxWorkers = 2;
+const defaultMaxWorkers = Math.max(1, cpus().length - 1);
 const requestedMaxWorkers = Number.parseInt(process.env.VITEST_MAX_WORKERS ?? String(defaultMaxWorkers), 10);
-const maxWorkers = Math.max(1, Math.min(4, Number.isFinite(requestedMaxWorkers) ? requestedMaxWorkers : defaultMaxWorkers));
+const maxWorkers = Math.max(1, Number.isFinite(requestedMaxWorkers) ? requestedMaxWorkers : defaultMaxWorkers);
 process.env.VITEST_MAX_WORKERS = String(maxWorkers);
 const coreSourceEntry = fileURLToPath(new URL("../core/src/index.ts", import.meta.url));
 const testUtilsEntry = fileURLToPath(new URL("../core/src/__test-utils__/workspace.ts", import.meta.url));