feat(db): 新增 0002_memory 迁移；迁移测试改用嵌入式迁移集合（数量无关，避免卡死后续迁移）

internal/db/migrations/0002_memory.up.sql

@@ -0,0 +1,21 @@
+-- M2: long-term memory items (DevDocs/design/03 §0/§3).
+-- Canonical schema reference; internal/runtime/memory reuses this table.
+CREATE TABLE IF NOT EXISTS memory_items (
+    id                  TEXT PRIMARY KEY,
+    persona_id          TEXT NOT NULL,
+    role_id             TEXT NOT NULL DEFAULT '',
+    scope               TEXT NOT NULL,
+    kind                TEXT NOT NULL,
+    content             TEXT NOT NULL,
+    tags                TEXT NOT NULL DEFAULT '[]',
+    confidence          REAL NOT NULL DEFAULT 1.0,
+    source_conversation TEXT NOT NULL DEFAULT '',
+    created_at          TEXT NOT NULL,
+    updated_at          TEXT NOT NULL,
+    decay_weight        REAL NOT NULL DEFAULT 1.0
+);
+
+CREATE INDEX IF NOT EXISTS idx_memory_items_persona_id
+    ON memory_items (persona_id);
+CREATE INDEX IF NOT EXISTS idx_memory_items_persona_id_created_at
+    ON memory_items (persona_id, created_at);