fix(gateway): surface an 'unknown' toolchain status at the gate instead of passing silently

The toolchain guard fails open on a recorded 'unknown' status (precision over recall — never strand a task on an inconclusive smoke). But an 'unknown' means provisioning ran yet the smoke could not confirm the suite is collectable under the interpreter, so the gate was proceeding blind with no trace — a silent hollow pass. It now emits a 'toolchain.unverified_gate_pass' warning with the agent and task ids when proceeding past 'unknown', while still not blocking; a missing marker (None) stays silent so the warning carries signal.

tests/unit/gateway/test_toolchain_guard.py

@@ -15,6 +15,7 @@ from uuid import uuid4
 import pytest
 from roboco.config import settings
 from roboco.services.gateway.choreographer import Choreographer, ChoreographerDeps
+from structlog.testing import capture_logs
 
 
 def _make_choreographer(*, status: str | None) -> Choreographer:
@@ -68,3 +69,27 @@ async def test_guard_inert_when_flag_off(monkeypatch: pytest.MonkeyPatch) -> Non
     assert await c._toolchain_broken_guard(uuid4(), MagicMock()) is None
     # Flag off => the workspace is never consulted at all.
     c.git.toolchain_status_for_task.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_guard_warns_loudly_on_unknown(monkeypatch: pytest.MonkeyPatch) -> None:
+    # 'unknown' still fails open (never strands a task), but it must not be
+    # silent — a warning is emitted so the hollow pass is visible to operators.
+    monkeypatch.setattr(settings, "toolchain_match_enabled", True)
+    c = _make_choreographer(status="unknown")
+    with capture_logs() as logs:
+        env = await c._toolchain_broken_guard(uuid4(), MagicMock())
+    assert env is None
+    assert any(e.get("event") == "toolchain.unverified_gate_pass" for e in logs)
+
+
+@pytest.mark.asyncio
+async def test_guard_silent_when_no_marker(monkeypatch: pytest.MonkeyPatch) -> None:
+    # No marker (None) is benign — flag on but not yet provisioned / not a test
+    # project — and must stay silent so the warning means something.
+    monkeypatch.setattr(settings, "toolchain_match_enabled", True)
+    c = _make_choreographer(status=None)
+    with capture_logs() as logs:
+        env = await c._toolchain_broken_guard(uuid4(), MagicMock())
+    assert env is None
+    assert not any(e.get("event") == "toolchain.unverified_gate_pass" for e in logs)