fix(ci): exclude tests/docker/ from regular test shards; pin read_text encoding

Two CI follow-ups to @benbarclay's #30136 salvage:

1. scripts/run_tests_parallel.py — add 'docker' to _SKIP_PARTS so
   the new tests/docker/ harness doesn't run in the regular test (N)
   matrix. The harness builds the real Dockerfile in a session
   fixture, which can exceed pytest-timeout's 180s ceiling on
   ubuntu-latest where Docker IS available — it surfaced as 6
   identical setup-timeout failures across slices 1–6 on the first
   CI run.

   The docker harness has its own dedicated runner via
   .github/actions/hermes-smoke-test (added in #30136) plus the
   docker-lint workflow. Same treatment as tests/integration/ and
   tests/e2e/ — runs separately, not in the main shards.

2. hermes_cli/service_manager.py — pin encoding='utf-8' on the
   /proc/1/comm read_text call. Ruff PLW1514 enforcement rolled in
   between Ben's last push and the salvage; pure ruff-fix, no
   behavior change.

hermes_cli/service_manager.py

@@ -143,7 +143,7 @@ def _s6_running() -> bool:
     init, or an unrelated process named ``s6-svscan``).
     """
     try:
-        comm = Path("/proc/1/comm").read_text().strip()
+        comm = Path("/proc/1/comm").read_text(encoding="utf-8").strip()
     except OSError:
         return False
     if comm != "s6-svscan":