fix: resolve pip detection inconsistency in non-interactive shells

- Convert virtual environment Python paths to absolute paths to ensure
  consistency across different shell environments (Git Bash, WSL, etc.)
- Add enhanced diagnostic information when pip detection fails to help
  users troubleshoot path and environment issues
- Improve error messages with specific guidance for different platforms
- Fix black configuration to exclude .zen_venv directory from formatting
- Add comprehensive test suite for pip detection edge cases

Fixes #188

pyproject.toml

@@ -39,6 +39,7 @@ extend-exclude = '''
   | \.mypy_cache
   | \.tox
   | \.venv
+  | \.zen_venv
   | venv
   | _build
   | buck-out