feat: make Ollama/gemma4 the primary backend with Claude as fallback

Rolls back the claudification backend preference: PREFER_CLOUD_BACKEND now
defaults to false, resolve_backend() picks Ollama first and uses Claude when
explicitly preferred or when the new Ollama startup health check fails. The
Steward retries mid-request failures on the other backend in both directions.

Also hardens the fallback itself: Anthropic SDK imports are lazy so a broken
anthropic package degrades to Ollama-only instead of crashing at import time
(root cause of the production outage since April), anthropic is pinned to a
pydantic-ai-1.27-compatible range, ANTHROPIC_MODEL defaults to claude-sonnet-5
(sonnet-4-20250514 retired 2026-06-15), sampling parameters are stripped from
Claude calls (Sonnet 5 rejects them), and the Steward timeout is configurable
(STEWARD_TIMEOUT, default 60s) since gemma4 needs ~35s warm for analysis.

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

tests/agents/test_tatlock_agent.py

@@ -410,10 +410,12 @@ async def test_tatlock_ollama_fallback(async_client: AsyncClient):
             "stream": False
         }
 
+        # 300s: this test forbids the Claude rescue, and the full local
+        # Steward -> orchestrate -> synthesize flow on gemma4 exceeds 120s
         response = await async_client.post(
             "/v1/chat/completions",
             json=request_data,
-            timeout=120.0
+            timeout=300.0
         )
 
         assert response.status_code == 200