Track Windows agent processes after shell wrapper exit

## Problem

On Windows, ACP agents start through a shell wrapper and cleanup uses `taskkill /T /F` while that wrapper is alive. If the wrapper exits before cleanup starts, Windows no longer lets `taskkill` address the old tree by the wrapper PID. A custom command that detaches an ACP descendant can therefore leave it running.

PR #70 keeps real `taskkill` failures visible, but treats a natural wrapper exit as complete so one normal agent exit cannot reserve session capacity forever.

## Proposed direction

Track each launched agent with a Windows Job Object or another stable tree handle. Cleanup should terminate and wait for the tracked tree even after the shell wrapper exits.

## Acceptance criteria

- Cleanup can stop descendants after wrapper exit.
- PID reuse cannot target an unrelated process.
- Natural exits release session capacity after the tree is verified empty.
- Include a real Windows regression test with a wrapper that exits before its descendant.

Start by tracing the Windows agent launch and cleanup paths described in the issue, including the behavior referenced by PR #70. Verify how the wrapper and descendants are tracked, then add a real Windows regression test where the wrapper exits first; done means descendants can be stopped safely, PID reuse is avoided, and verified-empty trees release session capacity.

Written by the indexing model from the issue text.