Satisfy the CI F-rule lint gate: mark the re-export, drop dead test imports

PR #458 quick-test failed at the ruff F-rule step (a lane the local sprint
runs never exercised — my miss; the hermetic commit gate mirrors it but
these commits were made with plain git):
- ouroboros/llm.py: the provider_catalogs re-export is intentional
  surface, marked noqa: F401 like every other re-export seam in the repo
  (agent_task_pipeline, delegate_custody, headless, loop).
- tests/test_deepseek_provider.py: os and pytest were imported but never
  used.

ruff check . --select F: All checks passed. deepseek suite 30 passed;
size lane vs PR base ad595274 still 5 passed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ouroboros/llm.py

@@ -493,7 +493,7 @@ def add_usage(total: Dict[str, Any], usage: Dict[str, Any]) -> None:
 # Live pricing-catalog fetchers moved whole to ouroboros/provider_catalogs.py
 # at the 200,000-byte module ratchet ceiling; re-exported here so existing
 # importers (ouroboros.pricing, tests) keep the historical names.
-from ouroboros.provider_catalogs import (  # noqa: E402
+from ouroboros.provider_catalogs import (  # noqa: F401,E402
     fetch_cloudru_pricing,
     fetch_openrouter_pricing,
 )

tests/test_deepseek_provider.py

@@ -13,10 +13,6 @@ probes (2026-09-01):
   an explicit empty string satisfies the gate for turns produced elsewhere).
 """
 
-import os
-
-import pytest
-
 from ouroboros import provider_models
 from ouroboros.llm import LLMClient
 from ouroboros.provider_models import (