fix(kaos): terminate local process trees

[lihailong00]

Summary

• isolate every local KAOS command in its own process group/session
• terminate the exact local process tree on cancellation or timeout
• keep SSH and ACP implementations unchanged

Reproduction and verification

On Windows Git Bash, the regression test deterministically showed the old code killing only the Bash parent while a descendant survived and wait() timed out. The fixed test passed five consecutive runs.

• uv run pytest packages/kaos/tests -q — 25 passed, 29 platform-skipped
• targeted Ruff and Pyright — passed

This supersedes closed #2327 and avoids its reviewed platform-detection mismatch by using os.name consistently at both process creation and termination.

Closes #2310

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.