Add command for creating a tag

Tags are like branches, but immutable. A command should be available for creating a new tag. The syntax `baronial tag {name} [RefSpec]` where HEAD is the implict `RefSpec` feels appropriate.

Start at the repository's existing command entry point and inspect how branch creation and RefSpec resolution are implemented. Add the tag command using the proposed `baronial tag {name} [RefSpec]` syntax, defaulting to HEAD, and verify that the created tag cannot be changed.

Written by the indexing model from the issue text.