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

tests/anthropic/test_model_selector.py

@@ -0,0 +1,92 @@
+"""
+Unit tests for backend selection (Ollama primary, Claude fallback).
+
+These tests set the cached health-check globals directly so they are
+deterministic regardless of which services are reachable.
+"""
+import pytest
+
+from src.anthropic import model_selector
+from src.core.config import config
+
+
+@pytest.fixture
+def local_first(monkeypatch):
+    """Baseline: local-first config, both backends healthy."""
+    monkeypatch.setattr(config, "PREFER_CLOUD_BACKEND", False)
+    monkeypatch.setattr(config, "ANTHROPIC_API_KEY", "sk-test-fake")
+    monkeypatch.setattr(model_selector, "_claude_available", True)
+    monkeypatch.setattr(model_selector, "_ollama_available", True)
+
+
+class TestResolveBackend:
+    def test_default_is_ollama(self, local_first):
+        assert model_selector.resolve_backend() == "ollama"
+
+    def test_prefer_cloud_config_selects_claude(self, local_first, monkeypatch):
+        monkeypatch.setattr(config, "PREFER_CLOUD_BACKEND", True)
+        assert model_selector.resolve_backend() == "claude"
+
+    def test_prefer_cloud_override_selects_claude(self, local_first):
+        assert model_selector.resolve_backend(prefer_cloud=True) == "claude"
+
+    def test_prefer_cloud_without_claude_falls_back_to_ollama(self, local_first, monkeypatch):
+        monkeypatch.setattr(config, "PREFER_CLOUD_BACKEND", True)
+        monkeypatch.setattr(model_selector, "_claude_available", False)
+        assert model_selector.resolve_backend() == "ollama"
+
+    def test_ollama_down_falls_back_to_claude(self, local_first, monkeypatch):
+        monkeypatch.setattr(model_selector, "_ollama_available", False)
+        assert model_selector.resolve_backend() == "claude"
+
+    def test_ollama_down_without_claude_stays_ollama(self, local_first, monkeypatch):
+        monkeypatch.setattr(model_selector, "_ollama_available", False)
+        monkeypatch.setattr(model_selector, "_claude_available", False)
+        assert model_selector.resolve_backend() == "ollama"
+
+    def test_unknown_ollama_state_counts_as_available(self, local_first, monkeypatch):
+        monkeypatch.setattr(model_selector, "_ollama_available", None)
+        assert model_selector.resolve_backend() == "ollama"
+
+
+class TestGetModel:
+    def test_ollama_backend_returns_openai_chat_model(self, local_first):
+        from pydantic_ai.models.openai import OpenAIChatModel
+
+        model = model_selector.get_model()
+        assert isinstance(model, OpenAIChatModel)
+        assert model.model_name == config.OLLAMA_DEFAULT_MODEL
+
+    def test_claude_backend_returns_anthropic_model(self, local_first):
+        from pydantic_ai.models.anthropic import AnthropicModel
+
+        model = model_selector.get_model(prefer_cloud=True)
+        assert isinstance(model, AnthropicModel)
+        assert model.model_name == config.ANTHROPIC_MODEL
+
+
+class TestToolChoiceSettings:
+    def test_ollama_forces_tool_choice(self, local_first):
+        settings = model_selector.get_tool_choice_settings()
+        assert settings.get("extra_body") == {"tool_choice": "required"}
+
+    def test_claude_uses_native_tool_choice(self, local_first, monkeypatch):
+        monkeypatch.setattr(config, "PREFER_CLOUD_BACKEND", True)
+        settings = model_selector.get_tool_choice_settings()
+        assert not settings.get("extra_body")
+
+
+class TestGetModelInfo:
+    def test_reports_ollama_primary(self, local_first):
+        info = model_selector.get_model_info()
+        assert info["backend"] == "ollama"
+        assert info["model"] == config.OLLAMA_DEFAULT_MODEL
+        assert info["ollama_available"] is True
+        assert info["claude_available"] is True
+        assert info["prefer_cloud"] is False
+
+    def test_reports_claude_when_ollama_down(self, local_first, monkeypatch):
+        monkeypatch.setattr(model_selector, "_ollama_available", False)
+        info = model_selector.get_model_info()
+        assert info["backend"] == "claude"
+        assert info["model"] == config.ANTHROPIC_MODEL