perf(test): cap vitest worker fan-out to keep dashboard responsive

On high-core dev machines (e.g. 28-core M-series), per-package vitest
defaulted to cpus().length - 1 workers (27), and `pnpm test` ran 4
workspace packages concurrently — easily 100+ vitest threads per sweep.
When the dashboard had agents running tests, 2+ concurrent sweeps would
saturate CPU and the UI became sluggish.

- Cap defaultMaxWorkers to min(6, cpus()-1) in cli/dashboard/desktop/
  mobile/plugin-sdk vitest configs (engine and core were already capped)
- Lower root `pnpm test` workspace-concurrency 4 → 2
- VITEST_MAX_WORKERS override still respected for explicit fast runs

Worst-case fan-out drops from ~108 workers to ~12 per `pnpm test`.
CI runners with fewer cores are unaffected (cap doesn't bind).

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

packages/plugin-sdk/vitest.config.ts

@@ -3,7 +3,8 @@ import { cpus } from "node:os";
 import { resolve } from "node:path";
 import { fileURLToPath } from "node:url";
 
-const defaultMaxWorkers = Math.max(1, cpus().length - 1);
+// Cap fan-out to 6 to avoid saturating high-core machines under workspace concurrency.
+const defaultMaxWorkers = Math.min(6, Math.max(1, cpus().length - 1));
 const requestedMaxWorkers = Number.parseInt(process.env.VITEST_MAX_WORKERS ?? String(defaultMaxWorkers), 10);
 const maxWorkers = Math.max(1, Number.isFinite(requestedMaxWorkers) ? requestedMaxWorkers : defaultMaxWorkers);
 process.env.VITEST_MAX_WORKERS = String(maxWorkers);