fix: add --version flag to CLI

[garvgupta2711]

What does this PR do?

Adds a --version flag to the Agentrace CLI.

Previously, running python -m agentrace.cli --version failed because a subcommand was required.

This uses argparse's built-in version action to print the package version.

Testing

• Added a regression test for --version
• All 30 tests pass with pytest

Closes #30

[github-actions]

Welcome, @garvgupta2711, and thanks for your first pull request to Agentrace.

A quick look at CONTRIBUTING.md covers how things are set up and run here.

I'll review this shortly. Ask anything in the thread if something is unclear.

[github-actions]

And it's in. Thanks @garvgupta2711.

Good, focused change. Exactly the kind of thing Agentrace needs.

If Agentrace is useful to you, a star goes a long way.

Happy to see more from you whenever you have the time.