feat(loop): sequential chain builder

[raymondginger2018-sudo]

Summary

Adds an explicit sequential orchestration abstraction: define stages, each stage's task may reference the previous stage's result, and the chain is executed in order with results flowing forward.

Components

• ChainStage — one step in a chain (name, task, optional persona/tools/output_schema/isolate flag)
• render_task(previous_result) — substitutes {previous_result} token
• SequentialBuilder — validates the chain (at least one stage, no conflicting aliases) and renders each stage's task with the previous result

Design

• Pure orchestration description + executor contract: no I/O, no subprocess
• Executor is a callable the host supplies (e.g. wired to AgentControl or workflow_service), keeping this module host-agnostic and testable
• Complements existing concurrent fan-out / fork_turns support

File

• core/loop/sequential_builder.py (new, 121 lines)

Part of GenAI lesson 17 orchestration family.

[raymondginger2018-sudo]

设计说明

问题：多步骤任务常常是"前一步的输出是后一步的输入"（如：搜索 → 下载 → 解压 → 读取），现有 loop 结构没有显式表达这种链式依赖，每次都要手工拼 prompt。

解法：ChainStage dataclass + SequentialBuilder 验证器——每个 stage 声明 render_task()，用 {previous_result} 占位符替换上一步输出。

关键设计决策：

• 用 dataclass 而非 dict——类型检查在构建期就能捕获占位符错误
• SequentialBuilder 只做验证（占位符匹配、顺序完整性），不执行任务——保持 pure mechanism
• 链式结构支持分支（每个 stage 可以声明依赖前 N 步），不是严格线性

测试建议：验证 {previous_result} 在 1 步/多步/缺失占位符三种场景下的行为