fix(presets): clean up detached agent processes

[mikemikimike]

Summary

• Track descendants launched by the preset agent, including helpers that create a new session.
• Clean up tracked descendants before terminating the agent and after normal agent completion.
• Add POSIX regressions for stop-time and normal-exit cleanup of detached helpers.

Fixes #4189

Tests

• uv run pytest src/tests -q (333 passed, 4874 skipped on Windows)
• uv run ruff check .
• uv run ruff format --check .
• uv run pyright -p .
• git diff --check