Add version number display to CLI help and --version flag

[DH-SuBaKu]

Summary

Implements version number detection with the following priority:

1. CMake cache variable (-DVERSION_NUMBER=...)
2. Environment variable (VERSION_NUMBER)
3. Git describe (git describe --tags --dirty --always)
4. Fallback to '0.0.0-dev'

The version is now displayed in:

• Help output (-h/--help)
• Version flag (--version)

Version detection normalizes git tags by removing 'v' prefix, stripping commit hashes, and cleaning dirty markers.

Fixes #309

Type of Change

• New feature (non-breaking change which adds functionality)

Test Plan

Ran the full test suite to verify the changes don't break existing functionality:

cd ~/playground/openzl/build
ctest

All ~1200 C/C++ unit tests passed successfully.

Ran the CLI integration test suite:

cd ~/playground/openzl/cli/tests
python3 cli_integration_tests.py ../../build/cli/zli

All 14 integration tests passed successfully.

Manually verified version display:

• ./build/cli/zli --help - shows version in help header
• ./build/cli/zli --version - displays version string

Test Configuration

• Compiler: GCC (default system compiler)
• Build type: Release
• Platform(s): Linux (WSL - Windows Subsystem for Linux)

[DH-SuBaKu]

Hey maintainers! I implemented version detection with three options:

1. Compile Macro (-DVERSION_NUMBER=...)
2. Environment variable (VERSION_NUMBER)
3. Git describe fallback

The environment variable option is for CI/CD convenience, but I'm happy to simplify
if you think it's overkill. Let me know your preference!