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>

.changeset/cap-vitest-fanout.md

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+"@runfusion/fusion": patch
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+Cap per-package Vitest worker fan-out to 6 (from `cpus().length - 1`) and lower the root `pnpm test` workspace concurrency from 4 to 2. On high-core developer machines this prevents `pnpm test` from spawning 100+ worker threads, which was saturating CPU and slowing the dashboard while agents ran tests. Override is still available via `VITEST_MAX_WORKERS`.