Add a --version flag to the CLI

[aidandaly24]

Add a --version flag that prints the current version and exits.

Acceptance Criteria

• Running agentcore-cli --version should print the version number (e.g. 0.1.0)
• The version should be read from pyproject.toml or a __version__ variable
• The flag should exit immediately after printing

Notes

This is a small, scoped change. Look at the existing CLI argument parsing to find where to add it.