feat(history): 会话消息持久化 + 主/次级会话（design/16）

- runtime/history: Store + HistoryStore 接口（Append/List 游标分页/Recent/DeleteSession/NewSubSession/ListSessions），主会话固定 web、次级 web:<uuid>
- db 迁移 0003: chat_sessions + chat_messages + idx_chat_msgs_session_created（事务幂等）
- gateway: GET/POST /sessions、GET /sessions/{id}/messages 热分页、DELETE /sessions/{id}（主不可删 403）；Deps.History 可空注入
- 编排层落库: session_persist.go（submitInternal 写用户消息、completeTurn 写助手消息，拒绝的消息不落库）
- WS: wsEvent 加 sessionId 定向 + history 回放帧（握手上行后回放主会话最新 50 条，thinking 剥离）
- wire.go 装配 History 到 gateway.Deps.History
- 端到端实测: POST /sessions 建次级 201 / GET /sessions 列表 / POST /chat 后 /messages 读到 user+assistant / DELETE 次级成功 / DELETE 主会话 403

internal/db/migrations/0003_chat_history.down.sql

@@ -0,0 +1,4 @@
+-- 0003_chat_history: rolls back the chat history tables created by the up migration.
+DROP TABLE IF EXISTS chat_messages;
+DROP INDEX IF EXISTS idx_chat_msgs_session_created;
+DROP TABLE IF EXISTS chat_sessions;

internal/db/migrations/0003_chat_history.up.sql

@@ -0,0 +1,31 @@
+-- 0003_chat_history: persistent chat sessions + message stream (DevDocs/design/16 §2.1, §3.1).
+-- chat_sessions records the persistent sessions: one fixed main session plus arbitrarily
+-- many secondary (sub) sessions. chat_messages is the raw, durable message stream in
+-- millisecond-timestamp cursor order. The two are the backend truth for the Web frontend;
+-- they are NOT memory (design/16 §3.2 separates memory from the raw stream).
+--
+-- Its canonical schema is owned by internal/runtime/history, which reuses these tables. The
+-- migration is the authoritative path; internal/runtime/history.NewStore re-issues the same
+-- DDL idempotently so the store is self-contained (matching memory/session).
+
+CREATE TABLE IF NOT EXISTS chat_sessions (
+    id         TEXT PRIMARY KEY,            -- main = fixed id ("web"); sub = "web:<uuid>"
+    type       TEXT NOT NULL,               -- 'main' | 'sub'
+    title      TEXT NOT NULL DEFAULT '',
+    created_at INTEGER NOT NULL             -- ms timestamp
+);
+
+CREATE TABLE IF NOT EXISTS chat_messages (
+    id         TEXT PRIMARY KEY,            -- message unique id (uuid)
+    session_id TEXT NOT NULL,               -- owning session (main or sub)
+    role       TEXT NOT NULL,               -- user / assistant / system
+    content    TEXT NOT NULL,               -- raw message body (incl. <p>/<action>)
+    mode       TEXT,                        -- behavior mode (optional, bubble render)
+    mood       TEXT,                        -- mood (optional)
+    image      TEXT,                        -- attachment base64/URL (optional)
+    thinking   TEXT,                        -- chain of thought (optional, stored, not echoed by default)
+    created_at INTEGER NOT NULL             -- ms timestamp (pagination cursor)
+);
+
+CREATE INDEX IF NOT EXISTS idx_chat_msgs_session_created
+    ON chat_messages (session_id, created_at DESC);