Add --version command-line option

Adds a --version flag that displays the current mxdev version.
The version is automatically derived from git tags via hatch-vcs
during build.

Changes:
- Import __version__ from auto-generated _version.py with ImportError fallback
- Add --version argument to argparse parser
- Add comprehensive test cases:
  - test_parser_version(): verifies --version prints version and exits cleanly
  - test_version_format(): validates version string format
- Update CHANGES.md with feature documentation

Example usage:
$ mxdev --version
mxdev 5.0.1

All 194 tests pass (192 existing + 2 new version tests).

Author: jensens

CHANGES.md

@@ -2,6 +2,8 @@
 
 ## 5.0.2 (2025-10-23)
 
+- Feature: Added `--version` command-line option to display the current mxdev version. The version is automatically derived from git tags via hatch-vcs during build. Example: `mxdev --version` outputs "mxdev 5.0.1" for releases or "mxdev 5.0.1.dev27+g62877d7" for development versions.
+  [jensens]
 - Fix #70: HTTP-referenced requirements/constraints files are now properly cached and respected in offline mode. Previously, offline mode only skipped VCS operations but still fetched HTTP URLs. Now mxdev caches all HTTP content in `.mxdev_cache/` during online mode and reuses it during offline mode, enabling true offline operation. This fixes the inconsistent behavior where `-o/--offline` didn't prevent all network activity.
   [jensens]
 - Improvement: Enhanced help text for `-n/--no-fetch`, `-f/--fetch-only`, and `-o/--offline` command-line options to better explain their differences and when to use each one.

src/mxdev/main.py

@@ -10,6 +10,12 @@
 from .processing import write
 from .state import State
 
+
+try:
+    from ._version import __version__
+except ImportError:
+    __version__ = "unknown (not installed)"
+
 import argparse
 import logging
 import sys
@@ -48,6 +54,11 @@
 )
 parser.add_argument("-s", "--silent", help="Reduce verbosity", action="store_true")
 parser.add_argument("-v", "--verbose", help="Increase verbosity", action="store_true")
+parser.add_argument(
+    "--version",
+    action="version",
+    version=f"%(prog)s {__version__}",
+)
 
 
 def supports_unicode() -> bool:

tests/test_main.py

@@ -93,6 +93,39 @@ def test_parser_verbose():
     assert args.verbose is True
 
 
+def test_parser_version(capsys):
+    """Test --version prints version and exits."""
+    from mxdev.main import __version__
+    from mxdev.main import parser
+
+    import pytest
+
+    with pytest.raises(SystemExit) as exc_info:
+        parser.parse_args(["--version"])
+
+    # Verify clean exit
+    assert exc_info.value.code == 0
+
+    # Verify output contains version string
+    captured = capsys.readouterr()
+    assert __version__ in captured.out
+    assert "." in captured.out  # Version has dots (X.Y.Z)
+
+
+def test_version_format():
+    """Test version format is valid."""
+    from mxdev.main import __version__
+
+    # Version should not be the fallback
+    assert __version__ != "unknown (not installed)"
+
+    # Version should contain dots (semantic versioning)
+    assert "." in __version__
+
+    # Version should be a string
+    assert isinstance(__version__, str)
+
+
 def test_supports_unicode_with_utf8():
     """Test supports_unicode returns True for UTF-8 encoding."""
     from mxdev.main import supports_unicode