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/services/gateway/server.go

@@ -114,6 +114,10 @@ type HistoryStore interface {
 	// cross-end realtime sync (design/16 §4.2).
 	AppendFull(ctx context.Context, msg history.Message) (history.Message, error)
 	List(ctx context.Context, sessionID string, before int64, limit int, platform string) ([]history.Message, error)
+	// ListAfter returns up to limit messages strictly newer than after in
+	// oldest-first order, for forward incremental catch-up after a reconnect
+	// (the "before" cursor can only walk backwards). platform filters as in List.
+	ListAfter(ctx context.Context, sessionID string, after int64, limit int, platform string) ([]history.Message, error)
 	Recent(ctx context.Context, sessionID string, n int) ([]history.Message, error)
 	ListPlatformLog(ctx context.Context, platform string, before int64, limit int) ([]history.Message, error)
 	ListAdapterLog(ctx context.Context, adapterID string, before int64, limit int) ([]history.Message, error)