[moz] feat: Runaway Guard per-task dispatch 上限

§15 Runaway Guard — per-task dispatch_count 上限，防止无限循环 dispatch

问题：mail/toolchain task 走 handler auto-working（跳过 claim），不受
claim_timeout 3 次重试兜底保护。如果反复 spawn 但永远到不了 done/failed，
会无限循环消耗资源（实际案例：2026-06-15 mention 重复投递事件）。

设计：
- tasks 表新增 dispatch_count 字段
- 每次 ticker 成功 dispatch 时递增
- dispatch_count >= 10 时自动标 failed（reason=runaway_guard）
- 覆盖所有非终态（pending/working/claimed）
- 参考 Hermes v0.13 §3 Per-Task 重试上限

改动文件：
- src/blackboard/db.py: _safe_add_column dispatch_count
- src/blackboard/models.py: Task dataclass 加 dispatch_count
- src/daemon/ticker.py: dispatch 递增 + _check_timeouts runaway guard
- docs/design/15-runaway-guard.md: 设计文档
- tests/integration/test_ticker_integration.py: E13 测试 3 个

测试：456 passed, 3 skipped

tests/integration/test_ticker_integration.py

@@ -543,3 +543,94 @@ class TestCheckTimeoutsUnified:
         reclaimed = ticker._check_timeouts(db_path)
 
         assert "t-review-dead" not in reclaimed
+
+
+# ---------------------------------------------------------------------------
+# E13: §15 Runaway Guard — per-task dispatch_count 上限
+# ---------------------------------------------------------------------------
+
+class TestRunawayGuard:
+    """E13: dispatch_count >= 10 → 自动标 failed（覆盖所有非终态）"""
+
+    @pytest.fixture
+    def guard_project(self, tmp_path):
+        """创建项目 + 任务"""
+        data_root = tmp_path / "projects"
+        registry = ProjectRegistry(data_root)
+        registry.create_project("guard-proj", "Guard Test", agents=["agent-a"])
+        db_path = data_root / "guard-proj" / "blackboard.db"
+        bb = Blackboard(db_path)
+        return registry, db_path, bb
+
+    def test_runaway_guard_triggers_working(self, guard_project):
+        """E13.1: working 状态 dispatch_count >= 10 → 标 failed"""
+        registry, db_path, bb = guard_project
+
+        bb.create_task(Task(
+            id="t-runaway", title="Runaway Task", status="working",
+            assigned_by="daemon", current_agent="agent-a",
+        ))
+
+        conn = bb._conn()
+        try:
+            conn.execute(
+                "UPDATE tasks SET dispatch_count = 10 WHERE id = ?", ("t-runaway",))
+            conn.commit()
+        finally:
+            conn.close()
+
+        ticker = Ticker(registry, tick_interval=30)
+        reclaimed = ticker._check_timeouts(db_path)
+
+        assert "t-runaway" in reclaimed
+        task = Queries(db_path).task_by_id("t-runaway")
+        assert task.status == "failed"
+
+    def test_runaway_guard_triggers_pending(self, guard_project):
+        """E13.2: pending 状态 dispatch_count >= 10 → 标 failed"""
+        registry, db_path, bb = guard_project
+
+        bb.create_task(Task(
+            id="t-pending-runaway", title="Pending Runaway", status="pending",
+            assigned_by="daemon",
+        ))
+
+        conn = bb._conn()
+        try:
+            conn.execute(
+                "UPDATE tasks SET dispatch_count = 10 WHERE id = ?",
+                ("t-pending-runaway",))
+            conn.commit()
+        finally:
+            conn.close()
+
+        ticker = Ticker(registry, tick_interval=30)
+        reclaimed = ticker._check_timeouts(db_path)
+
+        assert "t-pending-runaway" in reclaimed
+        task = Queries(db_path).task_by_id("t-pending-runaway")
+        assert task.status == "failed"
+
+    def test_runaway_guard_not_triggered(self, guard_project):
+        """E13.3: dispatch_count < 10 → 正常流程不受影响"""
+        registry, db_path, bb = guard_project
+
+        bb.create_task(Task(
+            id="t-normal", title="Normal Task", status="working",
+            assigned_by="daemon", current_agent="agent-a",
+        ))
+
+        conn = bb._conn()
+        try:
+            conn.execute(
+                "UPDATE tasks SET dispatch_count = 5 WHERE id = ?", ("t-normal",))
+            conn.commit()
+        finally:
+            conn.close()
+
+        ticker = Ticker(registry, tick_interval=30)
+        reclaimed = ticker._check_timeouts(db_path)
+
+        assert "t-normal" not in reclaimed
+        task = Queries(db_path).task_by_id("t-normal")
+        assert task.status == "working"