test: add pytest test suite for commit-ai

[Simons-holo]

Implements issue #20 - adds pytest test suite for commit-ai.

Changes

• Created commit-ai/test_commit.py with 8 test cases
• Tests cover: get_diff, get_branch, get_recent_commits, generate_commit_message, empty_diff, auto_commit

Test Cases

1. test_get_diff_staged - staged diff
2. test_get_diff_all - all diff
3. test_get_branch - branch detection
4. test_get_recent_commits - recent commits
5. test_generate_commit_message_conventional - conventional format
6. test_generate_commit_message_simple - simple format
7. test_empty_diff - empty diff handling
8. test_auto_commit - auto commit functionality

Acceptance Criteria

• 5+ test cases covering core functionality
• Tests use mocking for external APIs
• Uses pytest framework