fix(agents): hide async hook console window on Windows

[put4o]

Summary

Prevent asynchronous shell hooks from briefly opening a sh.exe console window on Windows.

Motivation

execCommandAsync() starts asynchronous hook commands using a detached sh process. On Windows, detached: true does not suppress the console window created for the child process, causing sh.exe to briefly flash during normal codemie-opencode usage.

Changes

• Add windowsHide: true to the spawn() options used by asynchronous shell hooks.
• Add regression coverage to ensure asynchronous hook processes continue to use the hidden-window option.

Testing

• npm run lint — passed
• npm run build — passed
• npm run commitlint:last — passed
• Regression test for windowsHide: true — passed
• npm run ci — completed with 6 pre-existing failures in shell-hooks-source.test.ts on native Windows
• The same 6 tests also fail on the unmodified baseline, confirming they are unrelated to this change

Breaking Changes

None.

Related Issues

Fixes #494