feat: write unit and integration tests for all features

[suenot]

Description

Add a comprehensive test suite covering all major features:

• Unit tests for each function (version bumping, diff processing, model selection, etc.)
• Integration tests for provider interactions (mocked)
• CI should block merges if tests fail

Currently there are no automated tests, which leads to regressions when new code is pushed.

Acceptance criteria

• cargo test passes in CI
• Coverage for: version management, git operations, config parsing, diff truncation, model jail logic