feat: integrate DB with Open/Close/Put/Delete/Get

- Add db.go with DB struct: Open/Close/Put/Delete/Get/GetDurableSequence/IsWriteStopped
- Add db_test.go with lifecycle, put/get, delete, recovery, and corruption tests
- Extract sentinel errors to errkit/ leaf package to break import cycle (wal -> go_kv)
- Update wal/sequence.go, wal/writer.go to import errkit instead of root package
- Root errors.go re-exports from errkit for backward compatibility

Phase 1 Wave 4d complete (T19).

.omo/boulder.json

@@ -8,7 +8,7 @@
       "plan_name": "phase1-wal",
       "status": "active",
       "started_at": "2026-06-12T05:09:32.588Z",
-      "updated_at": "2026-06-12T06:02:11.867Z",
+      "updated_at": "2026-06-12T06:09:22.858Z",
       "session_ids": [
         "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
       ],
@@ -120,6 +120,19 @@
           "status": "completed",
           "ended_at": "2026-06-12T06:02:11.867Z",
           "elapsed_ms": 16713
+        },
+        "todo:19": {
+          "task_key": "todo:19",
+          "task_label": "19",
+          "task_title": "DB integration: Open/Close/Put/Delete/Get",
+          "session_id": "opencode:ses_1459293beffeLZq5Ac22xWZfBy",
+          "agent": "Sisyphus-Junior",
+          "category": "deep",
+          "updated_at": "2026-06-12T06:09:22.858Z",
+          "started_at": "2026-06-12T06:07:14.120Z",
+          "status": "completed",
+          "ended_at": "2026-06-12T06:09:22.858Z",
+          "elapsed_ms": 128738
         }
       }
     }
@@ -127,7 +140,7 @@
   "active_plan": "/home/dailz/workspace/src/go-kv/.omo/plans/phase1-wal.md",
   "started_at": "2026-06-12T05:09:32.588Z",
   "status": "active",
-  "updated_at": "2026-06-12T06:02:11.867Z",
+  "updated_at": "2026-06-12T06:09:22.858Z",
   "session_ids": [
     "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
   ],
@@ -239,6 +252,19 @@
       "status": "completed",
       "ended_at": "2026-06-12T06:02:11.867Z",
       "elapsed_ms": 16713
+    },
+    "todo:19": {
+      "task_key": "todo:19",
+      "task_label": "19",
+      "task_title": "DB integration: Open/Close/Put/Delete/Get",
+      "session_id": "opencode:ses_1459293beffeLZq5Ac22xWZfBy",
+      "agent": "Sisyphus-Junior",
+      "category": "deep",
+      "updated_at": "2026-06-12T06:09:22.858Z",
+      "started_at": "2026-06-12T06:07:14.120Z",
+      "status": "completed",
+      "ended_at": "2026-06-12T06:09:22.858Z",
+      "elapsed_ms": 128738
     }
   },
   "agent": "atlas"

.omo/plans/phase1-wal.md

@@ -1412,7 +1412,7 @@ Max Concurrent: 6 (Wave 1b)
   **Commit**: YES
   - Message: `feat: implement MANIFEST stub and CURRENT file`
 
-- [ ] 19. DB integration: Open/Close/Put/Delete/Get
+- [x] 19. DB integration: Open/Close/Put/Delete/Get
 
   **What to do**:
   - 创建 `db.go`：`DB` 结构体和核心 API