Problem:

Users currently cannot mark specific points in history (like releases) with a permanent, human-readable name. They must rely on memorizing commit hashes.

Fix:

Implement pit tag to manage lightweight references in .pit/refs/tags/.

Technical Detail:

A tag is functionally identical to a branch, except it is expected not to move. We will create a new directory .pit/refs/tags and write the commit hash into files named after the tag. commands/checkout.py will need updates to resolve tags.

Behaviour:

• pit tag <name>
  Creates a reference pointing to the current commit.

• pit tag
  Lists all tags.

• pit checkout <tag>
  Switches HEAD to the commit pointed to by the tag.