feat(memtable): implement SkipList with lock-free reads

- memtable/skiplist.go: heap-backed SkipList with atomic.Pointer next pointers
- Mutex-protected writes, lock-free reads with release/acquire ordering
- Copy-on-insert and copy-on-return for key/value safety
- Forward iterator that skips pending entries
- Comprehensive tests: ordered insert, overwrite, pending hidden, concurrent -race

.omo/boulder.json

@@ -8,7 +8,7 @@
       "plan_name": "phase1-wal",
       "status": "active",
       "started_at": "2026-06-12T05:09:32.588Z",
-      "updated_at": "2026-06-12T05:26:58.955Z",
+      "updated_at": "2026-06-12T05:33:52.854Z",
       "session_ids": [
         "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
       ],
@@ -55,6 +55,19 @@
           "status": "completed",
           "ended_at": "2026-06-12T05:26:58.955Z",
           "elapsed_ms": 15585
+        },
+        "todo:8": {
+          "task_key": "todo:8",
+          "task_label": "8",
+          "task_title": "SkipList (mutex write + lock-free read)",
+          "session_id": "opencode:ses_145b2e10affeQnDhHreILvKkkW",
+          "agent": "Sisyphus-Junior",
+          "category": "deep",
+          "updated_at": "2026-06-12T05:33:52.854Z",
+          "started_at": "2026-06-12T05:32:48.230Z",
+          "status": "completed",
+          "ended_at": "2026-06-12T05:33:52.854Z",
+          "elapsed_ms": 64624
         }
       }
     }
@@ -62,7 +75,7 @@
   "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": "2026-06-12T05:26:58.955Z",
+  "updated_at": "2026-06-12T05:33:52.854Z",
   "session_ids": [
     "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
   ],
@@ -109,6 +122,19 @@
       "status": "completed",
       "ended_at": "2026-06-12T05:26:58.955Z",
       "elapsed_ms": 15585
+    },
+    "todo:8": {
+      "task_key": "todo:8",
+      "task_label": "8",
+      "task_title": "SkipList (mutex write + lock-free read)",
+      "session_id": "opencode:ses_145b2e10affeQnDhHreILvKkkW",
+      "agent": "Sisyphus-Junior",
+      "category": "deep",
+      "updated_at": "2026-06-12T05:33:52.854Z",
+      "started_at": "2026-06-12T05:32:48.230Z",
+      "status": "completed",
+      "ended_at": "2026-06-12T05:33:52.854Z",
+      "elapsed_ms": 64624
     }
   },
   "agent": "atlas"