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>

src/agents/housekeeper/agent.py

@@ -204,13 +204,13 @@ async def run_housekeeper(
     )
 
     try:
-        # Use temperature 0.1 for slight exploration
-        from pydantic_ai.settings import ModelSettings
+        # Temperature 0.1 for slight exploration (skipped on Claude backend)
+        from src.anthropic.model_selector import get_sampling_settings
 
         result = await agent.run(
             prompt,
             message_history=message_history,
-            model_settings=ModelSettings(temperature=0.1),
+            model_settings=get_sampling_settings(0.1),
         )
 
         logger.info(
@@ -266,13 +266,13 @@ async def run_housekeeper_stream(
     )
 
     try:
-        # Use temperature 0.1 for slight exploration
-        from pydantic_ai.settings import ModelSettings
+        # Temperature 0.1 for slight exploration (skipped on Claude backend)
+        from src.anthropic.model_selector import get_sampling_settings
 
         async with agent.run_stream(
             prompt,
             message_history=message_history,
-            model_settings=ModelSettings(temperature=0.1),
+            model_settings=get_sampling_settings(0.1),
         ) as response:
             async for delta in response.stream_text(delta=True):
                 yield delta