修复 Windows 后端退出：taskkill 结束进程树 + turn.completed 支持 (#108)

* fix(executor): handle turn.completed and terminate process tree on Windows

* fix: 修复代码审查发现的安全和资源泄漏问题

修复内容：
1. Windows 测试 taskkill 副作用：fake process 在 Windows 上返回 Pid()==0，避免真实执行 taskkill
2. taskkill PATH 劫持风险：使用 SystemRoot 环境变量构建绝对路径
3. stdinPipe 资源泄漏：在 StdoutPipe() 和 Start() 失败路径关闭 stdinPipe
4. stderr drain 并发语义：移除 500ms 超时，确保 drain 完成后再访问共享缓冲

测试验证：
- go test ./... -race 通过
- TestRunCodexTask_ForcesStopAfterTurnCompleted 通过
- TestExecutorSignalAndTermination 通过

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Co-Authored-By: SWE-Agent.ai <noreply@swe-agent.ai>

---------

Co-authored-by: cexll <evanxian9@gmail.com>
Co-authored-by: SWE-Agent.ai <noreply@swe-agent.ai>

codeagent-wrapper/signal_windows.go

@@ -0,0 +1,36 @@
+//go:build windows
+// +build windows
+
+package main
+
+import (
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+)
+
+// sendTermSignal on Windows directly kills the process.
+// SIGTERM is not supported on Windows.
+func sendTermSignal(proc processHandle) error {
+	if proc == nil {
+		return nil
+	}
+	pid := proc.Pid()
+	if pid > 0 {
+		// Kill the whole process tree to avoid leaving inheriting child processes around.
+		// This also helps prevent exec.Cmd.Wait() from blocking on stderr/stdout pipes held open by children.
+		taskkill := "taskkill"
+		if root := os.Getenv("SystemRoot"); root != "" {
+			taskkill = filepath.Join(root, "System32", "taskkill.exe")
+		}
+		cmd := exec.Command(taskkill, "/PID", strconv.Itoa(pid), "/T", "/F")
+		cmd.Stdout = io.Discard
+		cmd.Stderr = io.Discard
+		if err := cmd.Run(); err == nil {
+			return nil
+		}
+	}
+	return proc.Kill()
+}