Add tests for all commands

[ajianaz]

Description

Increase test coverage across all cora-cli commands and modules.

Acceptance Criteria

• Unit tests for all public functions in scanner.rs, reviewer.rs, formatter.rs, config.rs
• Integration tests for all CLI commands (review, scan, config, completion)
• Test edge cases: empty diffs, large files, API errors, malformed config
• Minimum 80% code coverage

Implementation Notes

• Use assert_cmd for CLI integration tests
• Use mockall or wire mocks for LLM API testing
• Add CI coverage reporting with tarpaulin or llvm-cov

[ajianaz]

Implemented in PR #24, merged to develop.