refactor(context): Phase2 上下文引擎升级 分层记忆+压缩+隔离 [v2026.07.19.4]

migrations/021_create_conversation_summaries.sql

@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS conversation_summaries (
+  conversation_id VARCHAR(64) NOT NULL,
+  summary_text TEXT NULL,
+  token_count INT NULL DEFAULT 0,
+  updated_at DATETIME(3) NULL,
+  PRIMARY KEY (conversation_id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;