Fix banner unicode crashes and add git repository checks to commands

[Huerte]

Type

• Bug fix
• New feature
• Documentation
• Refactor / internal

Changes

• banner.py: fixed unicode crashes on older Windows terminals, made all box borders consistently green, and added a friendly message when run outside a git repo instead of throwing an error.
• gitgo push, pull, state, undo, and resolve: added a check so running them outside a git repo prints a friendly error instead of crashing.
• tests: added new tests in test_banner.py for the non-git repo behavior and updated test mocks in test_resolve.py.

Checklist

• Tested locally and changes work as expected
• CHANGELOG.md updated under [Unreleased]
• README.md updated (if a command was added or changed)
• Tests added or updated (if applicable)
• No existing commands are broken (if they are, describe the impact above)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 90.19608% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pygitgo/utils/banner.py	87.80%	5 Missing ⚠️

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