Avoid pkg_resources import on stable CLI startup

[Mirochill]

Fixes #394.

Summary:

• Remove the import-time pkg_resources dependency from the stable CLI.
• Use importlib.metadata for package version lookup when available.
• Keep pkg_resources as a late fallback for older Python versions without importlib.metadata.
• Add regression coverage for the CLI version fallback behavior.

Validation:

• git diff --check HEAD~1..HEAD
• Not run locally: test suite, tox, linter, build/install check, CLI runtime check