Improve Windows installation diagnostics

[josephsenior]

Problem

Windows users occasionally encounter silent failures during installation when prerequisites
(Python ≥ 3.11, uv, or Scoop) are misconfigured. The current error messages don't provide
actionable next steps.

Proposed solution

• Add a grinta doctor command that checks:
  • Python version and virtual environment status
  • uv availability and version
  • PATH configuration for Scoop/Homebrew installs
  • Git availability and configuration
  • Terminal encoding (UTF-8 support)
• Output a clear report with ✅/❌ for each check and suggested fixes for failures
• Run grinta doctor automatically on first launch if no config file is found

Acceptance criteria

• grinta doctor runs on Windows, macOS, and Linux
• Each check outputs a human-readable status with fix suggestions
• CI includes a Windows runner that exercises the doctor command

[immorello]

I'd like to work on this — starting with the uv availability check, terminal encoding check, and Scoop/Homebrew PATH check in backend/cli/doctor/checks.py. I'll leave the Windows CI runner and auto-run-on-first-launch pieces for a follow-up, happy to discuss scope if that doesn't work for you.

[josephsenior]

Thanks @immorello — that scope works well. Please go ahead with the uv availability, terminal encoding, and Scoop/Homebrew PATH checks in backend/cli/doctor/checks.py.

Please keep the checks independently testable and include unit tests for the main success and failure cases where practical. Leaving the Windows CI runner and first-launch integration for a follow-up is completely fine.

When ready, open a PR and reference this issue with Refs #91. Happy to review it!

[josephsenior]

Thanks @immorello! I see your commit in your fork. Please open a Pull Request (PR) targeting josephsenior/Grinta-Coding-Agent whenever you're ready so we can review and merge it!

[immorello]

Hi @josephsenior. Yup, I cannot work on this full-time but I have already done the uv check and the encoding check. Probably tomorrow I'll be able to open the PR if it's ok for you