Add missing test coverage for git and prompt packages

[tapvt]

Problem

Two critical packages are missing test files entirely:

• internal/git/git.go - No git_test.go file
• internal/prompt/prompt.go - No prompt_test.go file

Impact

• No test coverage for git integration functionality
• No test coverage for prompt generation and formatting
• Reduces confidence in code quality and reliability
• Makes refactoring risky

Current Status

When running make test-unit, these packages show:

Required Actions

1. Create internal/git/git_test.go with comprehensive tests for:

   • Git repository detection
   • Branch name extraction
   • Status checking (staged, unstaged, untracked files)
   • Ahead/behind commit counting
   • Error handling

2. Create internal/prompt/prompt_test.go with comprehensive tests for:

   • Prompt format string parsing
   • Git information integration
   • Color scheme application
   • Various prompt configurations

Files to Create

• internal/git/git_test.go
• internal/prompt/prompt_test.go

Priority

Critical - Test coverage is essential for code quality and maintenance.

Reference

See TODO.md item #3