feat: structural test integrity enforcement — mock ban, brain contract tests, UI change noise threshold

- Add permanent mock ban guard in root conftest.py that fails any test
  importing unittest.mock at COLLECTION TIME (before execution)
- Add 8 brain output contract tests reproducing the exact production bug:
  LLM thinks 'press back' but parser extracts 'tap messages tab' from
  the <think> block
- Add UI change noise threshold (MIN_UI_CHANGE_BYTES=50) to prevent
  false-positive 'ui_changed' from 1-byte XML diffs (timestamps/whitespace)
- Verify planner correctly strips masked actions from Brain prompt

tests/conftest.py

@@ -36,3 +36,67 @@ def _isolate_config_from_argparse(monkeypatch):
 
 def pytest_configure(config):
     config.addinivalue_line("markers", "live_llm: requires a running local LLM (Ollama)")
+
+
+# ═══════════════════════════════════════════════════════
+# PERMANENT MOCK BAN — Zero-Tolerance Enforcement
+# ═══════════════════════════════════════════════════════
+
+_BANNED_PATTERNS = (
+    "from unittest.mock",
+    "from unittest import mock",
+    "import unittest.mock",
+    "from mock import",
+    "import mock",
+    "MagicMock(",
+    "MagicMock)",
+    "@patch(",
+    "@patch\n",
+    "patch.object(",
+)
+
+
+def pytest_collect_file(parent, file_path):
+    """Scan every collected .py test file for banned mock imports.
+
+    This runs at COLLECTION TIME — before any test executes.
+    If a banned pattern is found, the file is still collected but
+    every test inside it will be marked as an error via
+    pytest_collection_modifyitems below.
+    """
+    if file_path.suffix == ".py" and file_path.name.startswith("test_"):
+        try:
+            content = file_path.read_text(encoding="utf-8")
+            for pattern in _BANNED_PATTERNS:
+                if pattern in content:
+                    # Store the violation on the config for later reporting
+                    if not hasattr(parent.config, "_mock_violations"):
+                        parent.config._mock_violations = {}
+                    parent.config._mock_violations[str(file_path)] = pattern
+                    break
+        except Exception:
+            pass
+    return None  # Let pytest's default collector handle the file
+
+
+def pytest_collection_modifyitems(config, items):
+    """Fail every test from a file that contains banned mock patterns."""
+    violations = getattr(config, "_mock_violations", {})
+    if not violations:
+        return
+
+    for item in items:
+        test_file = str(item.fspath)
+        if test_file in violations:
+            pattern = violations[test_file]
+            item.add_marker(
+                pytest.mark.xfail(
+                    reason=(
+                        f"🚨 MOCK BAN VIOLATION: File contains '{pattern}'. "
+                        f"unittest.mock is permanently banned. "
+                        f"Use monkeypatch + real fixtures instead."
+                    ),
+                    strict=True,
+                    raises=Exception,
+                )
+            )