feat(agent): step-level retry for transient LLM errors (20s/40s backoff)

核心算法 #10（Agent Loop）：仅加固异常路径的韧性兜底，不改变
解析协议、hook 时序与步数语义。benchmark 错题 796-3 显示一次
SSL BAD_RECORD_MAC 穿透 GovernedLLMClient 重试栈后废掉 13 步
已积累上下文；本次在 run() Phase 1 增加步级重试（默认 2 次，
20s/40s 退避），可重试异常限定 (TimeoutError, OSError)，非可
重试异常照旧 fail-fast 整题终止，行为与现状一致（P5 显式异常）。
为满足 radon C 级复杂度约束，重试循环抽取为私有方法
_call_llm_with_step_retry，行为不变。

tests/unit/test_agent_loop.py

@@ -8,6 +8,7 @@ thinking 捕获、token 累加、call_id 透传、pluggy hook。
 from __future__ import annotations
 
 import json
+import ssl
 from typing import TYPE_CHECKING, Any
 from unittest.mock import AsyncMock
 
@@ -305,3 +306,60 @@ class TestParseNormalization:
     def test_empty_content_still_rejected(self) -> None:
         """空 content 照旧拒绝，归一化不改变该边界。"""
         assert self._parse("") is None
+
+
+# ── A2 步级重试测试（Spec-1）──────────────────────────────────
+
+
+class TestStepLevelRetry:
+    """LLM 瞬时异常的步级重试：可重试元组 / 退避 / fail-fast。"""
+
+    def _make_loop(self, chat_side_effects: list) -> AgentLoop:
+        """构造 chat 按序抛异常/返回响应的 AgentLoop。"""
+        llm = AsyncMock()
+        llm.chat = AsyncMock(side_effect=chat_side_effects)
+        return AgentLoop(llm=llm, max_steps=10)
+
+    @pytest.mark.asyncio
+    async def test_transient_error_retried_then_succeeds(self, monkeypatch) -> None:
+        """SSL/超时瞬时异常按 20s/40s 退避重试，第三次成功 → finished。"""
+        delays: list[float] = []
+
+        async def _fake_sleep(seconds: float) -> None:
+            delays.append(seconds)
+
+        monkeypatch.setattr("core.agent.loop.asyncio.sleep", _fake_sleep)
+        loop = self._make_loop(
+            [
+                ssl.SSLError("SSLV3_ALERT_BAD_RECORD_MAC"),
+                TimeoutError("watchdog"),
+                _make_response(_submit_json()),
+            ]
+        )
+        result = await loop.run("sys", "user", _StubDispatcher())
+        assert result.stop_reason == "finished"
+        assert delays == [20.0, 40.0]
+
+    @pytest.mark.asyncio
+    async def test_retry_exhausted_terminates_with_error(self, monkeypatch) -> None:
+        """重试预算（首次 + 2 次）耗尽仍失败 → stop_reason=error。"""
+
+        async def _fake_sleep(seconds: float) -> None:
+            pass
+
+        monkeypatch.setattr("core.agent.loop.asyncio.sleep", _fake_sleep)
+        loop = self._make_loop([TimeoutError("t1"), TimeoutError("t2"), TimeoutError("t3")])
+        result = await loop.run("sys", "user", _StubDispatcher())
+        assert result.stop_reason == "error"
+        assert loop._llm.chat.await_count == 3  # 首次 + 2 次重试
+
+    @pytest.mark.asyncio
+    async def test_non_retryable_fails_fast(self, monkeypatch) -> None:
+        """非可重试异常不退避不重发，首次即终止 → stop_reason=error。"""
+        sleep_mock = AsyncMock()
+        monkeypatch.setattr("core.agent.loop.asyncio.sleep", sleep_mock)
+        loop = self._make_loop([RuntimeError("programming bug")])
+        result = await loop.run("sys", "user", _StubDispatcher())
+        assert result.stop_reason == "error"
+        sleep_mock.assert_not_awaited()
+        assert loop._llm.chat.await_count == 1