Add version flag to CLI

[ghanshyam2005singh]

• Rationale: Users expect kubectl a2a --version.
• Scope: Add @click.version_option() with fallback for editable installs.
• Files: src/cli.py
• Tasks:
  • Import importlib.metadata and wire version("kubestellar").
  • Gracefully fall back to "dev" when metadata absent.
  • Add unit test via Click CliRunner.
• Done: kubectl a2a --version prints semantic version; tests pass.

[ghanshyam2005singh]

/assign