fix: eliminate thinking-block poisoning + no-op navigation trap

ROOT CAUSE: qwen3.5 (reasoning model) returns response='' with thinking
block containing all reasoning. llm_provider.py line 352 silently
substituted the thinking block as the response via:
  content = raw_response or raw_thinking or ''
The Brain then extracted random actions from the reasoning text.

FIXES:
1. llm_provider.py: Conditional thinking isolation
   - format_json=True (SAE/perception): thinking fallback preserved
   - format_json=False (Brain): thinking NEVER substituted
   - Added think=false for Ollama free-text calls to force direct response

2. planner.py: No-Op Guard strips tab actions that navigate to
   the current screen (e.g. 'tap profile tab' on OWN_PROFILE)

3. test_brain_live.py: Stochastic testing (5 runs, 60% min valid)
   to handle non-deterministic LLM behavior reliably

4. tests/integration/test_llm_provider_pipeline.py: NEW test layer
   mocking at HTTP level (requests.post) to exercise the FULL
   llm_provider → Brain pipeline. This would have caught the
   thinking substitution bug from day one.

Suite: 168 passed, 0 failed

tests/unit/test_brain_output_contract.py

@@ -207,3 +207,130 @@ class TestUIChangedFidelity:
 
 
 from GramAddict.core.perception.screen_identity import ScreenType  # noqa: E402
+
+
+class TestBrainEmptyResponse:
+    """Contract: When the LLM returns response='', the Brain must NOT
+    extract actions from the thinking block. The thinking block is
+    REASONING, not decisions."""
+
+    def test_empty_response_returns_none_not_thinking_extraction(self, monkeypatch):
+        """REGRESSION: In the 2026-04-28 23:51 run, the LLM returned response=''
+        with a thinking block mentioning 'tap profile tab'. The Brain extracted
+        'tap profile tab' which was a no-op on OWN_PROFILE."""
+        import GramAddict.core.navigation.brain
+
+        def mock_llm(**kwargs):
+            # The EXACT production failure: response is empty, thinking has actions
+            return {"response": ""}
+
+        monkeypatch.setattr(GramAddict.core.navigation.brain, "query_llm", mock_llm)
+
+        result = ask_brain_for_action(
+            goal="nurture community",
+            screen_type="OWN_PROFILE",
+            available_actions=["tap message button", "scroll down", "press back", "tap profile tab"],
+            explored_actions=set(),
+        )
+        # When the LLM gives NO response, the Brain must return None
+        # to force the planner's structural fallback
+        assert result is None, (
+            f"Brain returned '{result}' from an empty LLM response! "
+            f"It must return None so the planner can use HD Map fallback."
+        )
+
+    def test_whitespace_only_response_treated_as_empty(self, monkeypatch):
+        """Response with only whitespace/newlines is effectively empty."""
+        import GramAddict.core.navigation.brain
+
+        def mock_llm(**kwargs):
+            return {"response": "  \n  \n  "}
+
+        monkeypatch.setattr(GramAddict.core.navigation.brain, "query_llm", mock_llm)
+
+        result = ask_brain_for_action(
+            goal="open explore",
+            screen_type="HOME_FEED",
+            available_actions=["tap explore tab", "scroll down"],
+            explored_actions=set(),
+        )
+        assert result is None, (
+            f"Brain returned '{result}' from a whitespace-only response! "
+            f"Must return None."
+        )
+
+
+class TestPlannerNoOpGuard:
+    """Contract: The planner must NEVER ask the Brain to execute a tab action
+    that would navigate to the screen we're already on."""
+
+    def test_planner_strips_current_screen_tab_before_brain(self, monkeypatch):
+        """On OWN_PROFILE, 'tap profile tab' is a no-op. The planner must
+        strip it from available_actions before asking the Brain."""
+        import GramAddict.core.navigation.brain
+        from GramAddict.core.navigation.planner import GoalPlanner
+
+        captured_prompts = []
+
+        def spy_query_llm(**kwargs):
+            captured_prompts.append(kwargs.get("system", ""))
+            return {"response": "scroll down"}
+
+        monkeypatch.setattr(GramAddict.core.navigation.brain, "query_llm", spy_query_llm)
+
+        planner = GoalPlanner("test_user")
+        screen = {
+            "screen_type": ScreenType.OWN_PROFILE,
+            "available_actions": ["tap profile tab", "tap home tab", "scroll down", "press back"],
+            "context": {},
+        }
+
+        planner.plan_next_step("nurture community", screen)
+
+        assert len(captured_prompts) == 1, "Brain was not called"
+        prompt = captured_prompts[0]
+
+        for line in prompt.splitlines():
+            if "available to you right now" in line:
+                assert "tap profile tab" not in line, (
+                    f"Planner passed no-op action 'tap profile tab' to Brain on OWN_PROFILE!\n"
+                    f"Line: {line}"
+                )
+                break
+        else:
+            pytest.fail("Could not find 'available to you right now' in Brain prompt")
+
+    def test_planner_strips_home_tab_on_home_feed(self, monkeypatch):
+        """On HOME_FEED, 'tap home tab' is a no-op."""
+        import GramAddict.core.navigation.brain
+        from GramAddict.core.navigation.planner import GoalPlanner
+
+        captured_prompts = []
+
+        def spy_query_llm(**kwargs):
+            captured_prompts.append(kwargs.get("system", ""))
+            return {"response": "scroll down"}
+
+        monkeypatch.setattr(GramAddict.core.navigation.brain, "query_llm", spy_query_llm)
+
+        planner = GoalPlanner("test_user")
+        screen = {
+            "screen_type": ScreenType.HOME_FEED,
+            "available_actions": ["tap home tab", "tap explore tab", "scroll down"],
+            "context": {},
+        }
+
+        planner.plan_next_step("nurture community", screen)
+
+        assert len(captured_prompts) == 1, "Brain was not called"
+        prompt = captured_prompts[0]
+
+        for line in prompt.splitlines():
+            if "available to you right now" in line:
+                assert "tap home tab" not in line, (
+                    f"Planner passed no-op 'tap home tab' to Brain on HOME_FEED!\n"
+                    f"Line: {line}"
+                )
+                break
+        else:
+            pytest.fail("Could not find 'available to you right now' in Brain prompt")