fix: Windows spawn EINVAL with paths containing spaces

[matiassanchez12]

Summary

Related Issue

Closes #

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Changes Made

Testing

• I have tested this locally
• I have added/updated tests (if applicable)
• All existing tests pass (npm test)

Checklist

• My code follows the existing code style
• I have not included version bumps (maintainer handles versioning)
• I have updated documentation if needed
• This PR focuses on a single feature/fix

Screenshots (if applicable)

Additional Notes

Fixes Windows spawn EINVAL errors when OpenCode is installed in paths with spaces (e.g. C:\Program Files).

Errors fixed:

1. `spawn EINVAL` - spawn() without `shell: true` cannot execute .cmd shims on Windows
2. `"C:\Program" is not recognized as an internal or external command` - `shell: true` without quoting paths with spaces causes Windows to split the command

[RoundTable02]

Enabling shell: true unconditionally makes this fix platform-wide. The Windows command is quoted above, but POSIX paths returned by resolveOpencodeCommand are now passed through /bin/sh unquoted and can still be split if they contain spaces. Please keep direct spawn(command, args, ...) behavior on POSIX and only enable shell mode for Windows, with a regression test for a Windows command path containing spaces.