Bugfix: Summaries on Windows

[thomas-intramotev]

Summary

Fixes #13: since this commit moved the Summaries prompt string from stdin to a CLI arg, AI summaries have been broken on Windows.

That said: the real source of the problem seems to have been my last PR on this feature where I set shell: true for multiple child process calls on Windows. This evidently made Windows CMD unable to handle the newlines in the prompt string, and from what I can tell: it was never necessary in the first place, and might even have had security implications. Claude, ironically, led me wrong on that one 🙃

Changes

• Removes the shell: true child process argument in several places (all tested to have no impact on Windows)
• In a similar vein: strips the environment passed by StandaloneMessageHandler.ts to child processes, similar to how SummaryService.ts does it

Checklist

• npm test passes
• npm run lint passes
• Webview builds (cd webview && npm run build)
• Tested manually in the Extension Development Host
• Updated CHANGELOG.md (if user-facing change)
• User-facing strings use vscode.l10n.t() for i18n