feat(history): 消息正向增量拉取(A5) — GET /sessions/{id}/messages?after=

- history/list_after.go: ListAfter(升序, created_at>after, 走覆盖索引, rowid兜底同毫秒)
- HistoryStore 新增 ListAfter(store+fakes同步)
- gateway history.go: handleListMessages解析after分流; after+before互斥400; 负数/非数字400
- 新增测试(list_after/history_after), build/vet/test绿+交叉编译
- Docs/api-client v0.26 + api-reference §3.4 同步

internal/runtime/history/store.go

@@ -132,6 +132,11 @@ type HistoryStore interface {
 	// returned (the Web client requests the full main session, an IM adapter asks for
 	// its own platform only).
 	List(ctx context.Context, sessionID string, before int64, limit int, platform string) ([]Message, error)
+	// ListAfter returns up to limit messages for sessionID strictly newer than after
+	// (created_at > after) in oldest-first order, so a client that reconnects can pull
+	// the messages it missed forward from the newest createdAt it has already seen.
+	// platform filters as in List. limit is clamped to [1, maxPageLimit].
+	ListAfter(ctx context.Context, sessionID string, after int64, limit int, platform string) ([]Message, error)
 	// Recent returns the most recent n messages for sessionID in oldest-first order,
 	// for WS replay / context tail usage.
 	Recent(ctx context.Context, sessionID string, n int) ([]Message, error)