feat(grok): cost-ceiling kill-switch (budget-guardrail parity)

Claude Code's per-agent token-budget hook fires against the SDK :9000 server;
opencode exposes NO usage/budget hook to a plugin (confirmed against its plugin
docs), so the budget kill-switch can't be a plugin/sidecar — the orchestrator
enforces it instead.

_enforce_grok_cost_budget runs each dispatch tick: for every ACTIVE GROK
container it reads cumulative cost from the opencode store (the Phase-2 reader)
and kills + evicts it past ROBOCO_GROK_MAX_COST_USD (0 = off), after which the
reaper releases the freed task. This also catches a runaway loop that keeps
firing verbs (so it evades the idle watchdog) but still burns cost.

Covers the budget/runaway-burn slice of guardrail parity. The remaining Claude
hooks (prompt-injection PRE-gate, stop-guard terminal-verb) have no blocking
opencode equivalent — opencode's message/stop hooks are observe-only — and the
interactive reasoning-variant has no opencode.json/serve knob (CLI-flag only);
both are pinned for a live probe rather than shipped as a guess.

tests/unit/runtime/test_grok_cost_budget.py

@@ -0,0 +1,83 @@
+"""GROK cost budget kill-switch: kill a live container over the cost ceiling.
+
+opencode exposes no usage hook to a plugin, so the budget kill-switch lives in
+the orchestrator: it reads each live GROK container's cumulative opencode cost
+and kills + evicts it past ROBOCO_GROK_MAX_COST_USD (also catching runaway-loop
+token burn). The cost computation itself is covered in opencode_usage tests; here
+cost_for_session is stubbed so the kill DECISION is exercised deterministically.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import AsyncMock
+
+import pytest
+from roboco.models.runtime import AgentInstance
+from roboco.runtime.orchestrator import AgentOrchestrator, AgentState
+
+_COST_FN = "roboco.llm.providers.opencode_usage.cost_for_session"
+
+
+def _grok_instance(provider_type: str = "grok") -> AgentInstance:
+    cfg = type("C", (), {"provider_type": provider_type, "model": "grok-build-0.1"})()
+    return AgentInstance(agent_id="be-dev-1", state=AgentState.ACTIVE, config=cfg)
+
+
+@pytest.mark.asyncio
+async def test_cost_over_cap_kills_and_evicts(monkeypatch: pytest.MonkeyPatch) -> None:
+    orch = AgentOrchestrator.__new__(AgentOrchestrator)
+    orch._grok_max_cost_usd = 5.0
+    orch._instances = {"be-dev-1": _grok_instance()}
+    remove_mock = AsyncMock()
+    monkeypatch.setattr(orch, "_remove_container", remove_mock)
+    monkeypatch.setattr(_COST_FN, lambda *_a, **_k: (None, 7.5))
+
+    await orch._enforce_grok_cost_budget()
+
+    remove_mock.assert_awaited_once_with("roboco-agent-be-dev-1")
+    assert "be-dev-1" not in orch._instances
+
+
+@pytest.mark.asyncio
+async def test_cost_under_cap_spares(monkeypatch: pytest.MonkeyPatch) -> None:
+    orch = AgentOrchestrator.__new__(AgentOrchestrator)
+    orch._grok_max_cost_usd = 5.0
+    orch._instances = {"be-dev-1": _grok_instance()}
+    remove_mock = AsyncMock()
+    monkeypatch.setattr(orch, "_remove_container", remove_mock)
+    monkeypatch.setattr(_COST_FN, lambda *_a, **_k: (None, 1.0))
+
+    await orch._enforce_grok_cost_budget()
+
+    remove_mock.assert_not_awaited()
+    assert "be-dev-1" in orch._instances
+
+
+@pytest.mark.asyncio
+async def test_cap_zero_disables_the_sweep(monkeypatch: pytest.MonkeyPatch) -> None:
+    orch = AgentOrchestrator.__new__(AgentOrchestrator)
+    orch._grok_max_cost_usd = 0.0
+    orch._instances = {"be-dev-1": _grok_instance()}
+    remove_mock = AsyncMock()
+    monkeypatch.setattr(orch, "_remove_container", remove_mock)
+    monkeypatch.setattr(_COST_FN, lambda *_a, **_k: (None, 999.0))
+
+    await orch._enforce_grok_cost_budget()
+
+    remove_mock.assert_not_awaited()
+    assert "be-dev-1" in orch._instances
+
+
+@pytest.mark.asyncio
+async def test_non_grok_container_is_ignored(monkeypatch: pytest.MonkeyPatch) -> None:
+    orch = AgentOrchestrator.__new__(AgentOrchestrator)
+    orch._grok_max_cost_usd = 5.0
+    orch._instances = {"be-dev-1": _grok_instance(provider_type="anthropic")}
+    remove_mock = AsyncMock()
+    monkeypatch.setattr(orch, "_remove_container", remove_mock)
+    monkeypatch.setattr(_COST_FN, lambda *_a, **_k: (None, 999.0))
+
+    await orch._enforce_grok_cost_budget()
+
+    remove_mock.assert_not_awaited()
+    assert "be-dev-1" in orch._instances