fix(executor): agent output truncated — cmd.Wait closes pipes before scanners drain

[liuxiaocs7]

Agent executor silently truncates output and loses the session ID

AgentExecutor.Run (internal/executor/agent.go) called cmd.Wait() while goroutines were still reading cmd.StdoutPipe() / cmd.StderrPipe(). Per the StdoutPipe contract, Wait closes the parent pipe ends as soon as the process exits — it is incorrect to call it before all reads complete. Output the scanners had not read yet is dropped silently: no error, no log. Under load the entire output is lost, including the session_id line.

中文描述

AgentExecutor.Run 在扫描 goroutine 还没读完 StdoutPipe/StderrPipe 时就调用了 cmd.Wait()。按 StdoutPipe 的文档约定，进程一退出 Wait 就会关闭父端管道——在读完成之前调用是错误的。扫描器尚未读到的输出会被静默丢弃：无报错、无日志。高负载下（如 CI 的 -race）全部输出都会丢失，包括 session_id 行，导致 agent 日志不完整、无法按 session 恢复。

Evidence

• CI (go test -race): TestAgentExecutorFakeCommand failed with session="" on 2/2 runs of PR feat(llm): support the OpenAI Responses API protocol #4, while the identical code passed locally and on main's single run — the classic signature of a scheduling race, not a functional failure.
• Minimal standalone repro of the same pattern (child echos 3 lines and exits, reader goroutine delayed 3 ms): old order lost the whole output in 60/100 runs; drain-first lost 0/100.

Impact

• Production: under load, an agent run (claude / cursor / codex) can lose output lines and the session ID — auto-dev logs are incomplete and resume-by-session becomes impossible. No error surfaces anywhere.
• CI: manifests as a flaky TestAgentExecutorFakeCommand failure that randomly blocks PRs.

Expected behavior

1. The scanners drain the full pipe contents before the pipes are closed — output and session ID are always captured regardless of scheduling.
2. A descendant process that inherits the pipe and lingers must not hang the run — bounded by a grace period, with a visible truncation note.

Acceptance criteria

• Session line captured even when the child exits immediately after emitting
• Grandchild output written after the parent exits is still captured
• A lingering descendant returns within the grace bound and logs a truncation note
• Regression tests fail against the old code and pass with the fix (go test -race ./internal/...)