refactor(tests): explicit HERMES_TEST_WORKERS over docker-suite auto-cap

Replace the runner's tests/docker auto-cap heuristic with explicit
width control:

- run_tests.sh forwards HERMES_TEST_WORKERS through its hermetic env -i
  (previously silently stripped — the documented override never worked
  through the wrapper)
- run_tests_parallel.py drops the _DOCKERD_BOUND_JOBS special-case; the
  suite-specific knowledge moves to the one place that runs that suite
- docker.yml pins HERMES_TEST_WORKERS=8. Width sweep with prewarmed
  image + split files: -j4 58-62s, -j8 39s, -j12 35s w/ ~2x per-file
  contention inflation; 8 is the knee.

Chain verified end-to-end: env var reaches the runner (6-worker probe),
no cap message on docker-only file lists, full suite 53/53 in 37.8s
at -j8.

.github/workflows/docker.yml

@@ -150,10 +150,16 @@ jobs:
           command: uv sync --locked --python 3.11 --extra dev
 
       - name: Run docker integration tests
+        # HERMES_TEST_WORKERS=8: this suite shares ONE dockerd, so width
+        # is daemon-bound, not CPU-bound. Width sweep with prewarmed image
+        # + split files (2026-07): -j4 58-62s, -j8 39s, -j12 35s but with
+        # ~2x per-file contention inflation at 12. 8 is the knee.
+        # run_tests.sh forwards HERMES_TEST_WORKERS through its hermetic
+        # env -i into the parallel runner.
         uses: ./.github/actions/profile
         with:
           label: docker-tests
-          command: HERMES_TEST_IMAGE="${{ env.IMAGE_NAME }}:test" scripts/run_tests.sh tests/docker/ --file-timeout 600
+          command: HERMES_TEST_WORKERS=8 HERMES_TEST_IMAGE="${{ env.IMAGE_NAME }}:test" scripts/run_tests.sh tests/docker/ --file-timeout 600
 
   # ---------------------------------------------------------------------------
   # Rebuild and push each architecture only after the unprivileged build/test