fix(persist,channel): 同类定宽时间戳修复 — checkpoint 恢复与渠道会话列表排序

- persist: formatTime 定宽；loadLatestSQL 补 rowid 兜底；New 时归一化历史行（避免 LoadLatest 挑到更旧检查点）
- channel/session: 写入定宽；listSessionSQL 补 rowid 兜底；核实无 INSERT 吃 DDL 默认值，故只加注释不改默认值
- 新增回归测试并做变异验证（改回 RFC3339Nano 即 FAIL）

internal/runtime/persist/persist_test.go

@@ -360,3 +360,67 @@ func TestStore_Save_MissingID_异常(t *testing.T) {
 		t.Fatalf("Save(blank id) err = %v, want wrap of ErrMissingID", err)
 	}
 }
+
+// TestStore_LoadLatest_变宽时间戳_取较晚 is the regression for the variable-width
+// timestamp bug on the durable-checkpoint recovery path. time.RFC3339Nano drops
+// trailing fractional zeros, so the older checkpoint was once stored as
+// ":05.83Z" and the later one as ":05.8309789Z". SQLite compares updated_at as
+// plain text, and "…83Z" ranks above "…8309789Z", so LoadLatest used to recover
+// the older checkpoint — a silent wrong resume. The older checkpoint also
+// carries the larger step, so the step DESC tiebreak cannot mask the bad order.
+func TestStore_LoadLatest_变宽时间戳_取较晚(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	s := newTestStore(t)
+
+	const (
+		olderRaw = "2026-09-07T03:20:05.83Z"      // X.8300000 under RFC3339Nano
+		newerRaw = "2026-09-07T03:20:05.8309789Z" // X.8309789 under RFC3339Nano
+	)
+	if _, err := s.db.ExecContext(ctx, insertLegacyCheckpointSQL,
+		"cp-older", "agent-a", 9, `{"step":9}`, olderRaw, olderRaw); err != nil {
+		t.Fatalf("insert older checkpoint: %v", err)
+	}
+	if _, err := s.db.ExecContext(ctx, insertLegacyCheckpointSQL,
+		"cp-newer", "agent-a", 2, `{"step":2}`, newerRaw, newerRaw); err != nil {
+		t.Fatalf("insert newer checkpoint: %v", err)
+	}
+
+	// Re-open so the fixed-width normalization repairs the legacy rows.
+	if _, err := New(ctx, s.db, testLogger()); err != nil {
+		t.Fatalf("New (normalize): %v", err)
+	}
+
+	got, err := s.LoadLatest(ctx, "agent-a")
+	if err != nil {
+		t.Fatalf("LoadLatest error: %v", err)
+	}
+	if got.ID != "cp-newer" {
+		t.Fatalf("LoadLatest = %q (step %d), want cp-newer (the later updated_at)", got.ID, got.Step)
+	}
+	if want := time.Date(2026, 9, 7, 3, 20, 5, 830978900, time.UTC); !got.UpdatedAt.Equal(want) {
+		t.Fatalf("LoadLatest updated_at = %v, want %v", got.UpdatedAt, want)
+	}
+}
+
+// TestStore_Save_定宽时间戳 pins that every write uses the fixed-width layout, so
+// text ordering equals time ordering for rows created by the current code.
+func TestStore_Save_定宽时间戳(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	s := newTestStore(t)
+
+	if err := s.Save(ctx, &Checkpoint{ID: "cp-1", AgentID: "agent-a", Step: 1, State: []byte(`{}`)}); err != nil {
+		t.Fatalf("Save error: %v", err)
+	}
+	var created, updated string
+	if err := s.db.QueryRowContext(ctx,
+		`SELECT created_at, updated_at FROM checkpoints WHERE id = ?`, "cp-1").
+		Scan(&created, &updated); err != nil {
+		t.Fatalf("read stored timestamps: %v", err)
+	}
+	if len(created) != storedTimeLen || len(updated) != storedTimeLen {
+		t.Errorf("stored timestamps = %q/%q (%d/%d bytes), want fixed width %d",
+			created, updated, len(created), len(updated), storedTimeLen)
+	}
+}