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/anthropic/__init__.py

@@ -1,19 +1,26 @@
 """
 Anthropic/Claude integration module.
 
-Provides model selection with automatic fallback between Claude and Ollama.
+Provides model selection with Ollama as primary backend and Claude
+as the cloud fallback.
 """
 
 from src.anthropic.model_selector import (
     check_claude_health,
+    check_ollama_health,
     get_model,
     get_tool_choice_settings,
     is_claude_available,
+    is_ollama_available,
+    resolve_backend,
 )
 
 __all__ = [
     "check_claude_health",
+    "check_ollama_health",
     "get_model",
     "get_tool_choice_settings",
     "is_claude_available",
+    "is_ollama_available",
+    "resolve_backend",
 ]