feat(ask): add default-dark ASK serving gate helper

Adds a default-dark ask_serving_enabled helper with focused tests. The helper is unwired: no pass_manager integration, no runtime surface, no carve-out retirement, and no served question text.

core/epistemic_questions/serving_gate.py

@@ -0,0 +1,32 @@
+"""ASK serving gate helper — default-dark, no served-surface wiring.
+
+This module is the first code slice after the ASK serving-integration scoping brief.
+It intentionally does **not** call ``deliver_ask``/``emit_question``, does not import
+``chat.runtime``, and does not expose any user-facing surface. It only centralizes the
+kill-switch read so future serving code has one audited predicate.
+
+The planned config field is ``RuntimeConfig.ask_serving_enabled``. During this dark-gate
+slice the predicate is conservative: absent field == ``False``. That lets the helper land
+without widening behavior and preserves the current default for every existing
+``RuntimeConfig`` instance.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from core.config import DEFAULT_CONFIG, RuntimeConfig
+
+
+def ask_serving_enabled(config: RuntimeConfig | Any | None = None) -> bool:
+    """Return whether served ASK delivery is explicitly enabled.
+
+    Missing attribute means ``False``. That is the load-bearing dark-gate invariant:
+    the served ASK path cannot light merely because the helper exists or because an
+    older ``RuntimeConfig`` instance lacks the future field.
+    """
+    cfg = DEFAULT_CONFIG if config is None else config
+    return bool(getattr(cfg, "ask_serving_enabled", False))
+
+
+__all__ = ["ask_serving_enabled"]

tests/test_ask_serving_gate.py

@@ -0,0 +1,43 @@
+"""ASK serving gate — default-dark invariant.
+
+This is deliberately narrower than serving integration. It proves the post-scoping
+kill-switch predicate is dark unless an operator/config object explicitly opts in.
+No chat/runtime wiring, no pass-manager emission, no carve-out retirement.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from core.config import DEFAULT_CONFIG, RuntimeConfig
+from core.epistemic_questions.serving_gate import ask_serving_enabled
+
+
+@dataclass(frozen=True, slots=True)
+class _LegacyConfig:
+    """A pre-field config shape: absence of the flag must mean dark."""
+
+    unrelated: bool = True
+
+
+@dataclass(frozen=True, slots=True)
+class _OptInConfig:
+    ask_serving_enabled: bool
+
+
+def test_default_runtime_config_keeps_ask_serving_dark() -> None:
+    assert ask_serving_enabled(DEFAULT_CONFIG) is False
+    assert ask_serving_enabled(RuntimeConfig()) is False
+
+
+def test_missing_flag_is_dark_for_legacy_config_shape() -> None:
+    assert ask_serving_enabled(_LegacyConfig()) is False
+
+
+def test_gate_only_lights_on_explicit_truthy_opt_in() -> None:
+    assert ask_serving_enabled(_OptInConfig(False)) is False
+    assert ask_serving_enabled(_OptInConfig(True)) is True
+
+
+def test_none_uses_default_config_and_stays_dark() -> None:
+    assert ask_serving_enabled() is False