fix(exec): kill child process tree on timeout to prevent orphaned tasks

[lunareed720]

Summary

This fixes orphaned background processes when exec commands time out.

What changed

• Run shell commands in a dedicated process group on Unix (Setpgid: true)
• On timeout/cancel, terminate the full process tree instead of only the parent shell
  • Unix: kill process group (kill -SIGKILL -pgid)
  • Windows: taskkill /T /F /PID fallback
• Switch ExecTool execution flow to Start + Wait with explicit timeout path and cleanup
• Add regression test (Unix): verifies timed-out command with spawned child (sleep) does not leave child process alive

Why

Issue #311 reports that failed/timed-out shell commands keep running in background. This can leak resources and degrade long-running agent stability.

Testing

• go test ./pkg/tools -run TestShellTool -count=1

Closes #311