Add pit tag Command To Give Readable Names To Commits

[BIJJUDAMA]

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.

[nimaldanyathk]

/assign

[Devpool-soc]

Alright @nimaldanyathk! Consider yourself officially assigned as now. If you are thinking, "Whoa, hold up, I didn't sign up for this," no worries, just yell "/unassign" and we can pretend this never happened. But if you are sticking around, just know your clock for this ticks out on: 2026-01-13. Don't screw it up :)