ci: bound profile e2e subprocess fan-out

Set DSH_E2E_MAX_WORKERS=4 for the credentialed e2e workflow and pin that environment contract in the workflow test. Profile-launched SDK and ACP scenarios each boot a complete subprocess tree, so the previous file-level fan-out could multiply process and provider pressure far beyond the runner's useful concurrency.

The bound changes scheduling only: every e2e file still runs, the Vitest configuration retains its explicit override knob, and local callers can choose a different positive worker count when their resources allow it.

.github/workflows/e2e.yml

@@ -60,8 +60,9 @@ jobs:
     if: >-
       github.event_name != 'pull_request'
       || !(github.event.pull_request.head.repo.fork || github.event.pull_request.user.login == 'dependabot[bot]')
-    # Bounded file parallelism (DSH_E2E_MAX_WORKERS), 120s/test, retry 2. 45m
-    # still bounds retry storms against a slow API while the happy path fans out.
+    # Profile e2e files can each own several complete dsh subprocess trees, so
+    # four file workers preserve process/PTY headroom. Tests retain 120s/test
+    # and retry 2; 45m still bounds retry storms against a slow API.
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v6
@@ -115,6 +116,6 @@ jobs:
         env:
           DEEPSEEK_API_KEY: ${{ secrets.DEEPSEEK_API_KEY_EXTERNAL }}
           DEEPSEEK_BASE_URL: https://api.deepseek.com
-          DSH_E2E_MAX_WORKERS: 14
+          DSH_E2E_MAX_WORKERS: 4
           DSH_EXAMPLE_MODE: lib
         run: pnpm run test:e2e