feat: never fetch tags after the initial clone

[dwilding]

This PR clarifies how gimmegit handles tags. Since gimmegit is for manipulating branches, all fetch operations including git update-branch specify --no-tags. If the user wants to clone tags, they can provide -- --tags to gimmegit, or they can manually fetch tags at any time using git fetch --tags.

[Copilot]

Pull request overview

This PR clarifies gimmegit's tag handling policy by documenting that tags are never fetched after the initial clone. Since gimmegit focuses on branch manipulation, all fetch operations now consistently use --no-tags, and users can only fetch tags during the initial clone by providing -- --tags or manually afterward.

Changes:

• Renamed "GIT OPTIONS" to "GIT CLONE OPTIONS" to clarify that the documented options apply specifically to the initial clone operation
• Removed TODO comments about replacing no_tags parameter, confirming the tag handling strategy
• Updated the update-branch alias to display --no-tags in its command output for transparency
• Refactored clone function to use native single_branch=True parameter instead of passing it as a string option

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/gimmegit/_help.py	Updated section header to "GIT CLONE OPTIONS" for clarity
src/gimmegit/_cli.py	Refactored clone to use native parameter, added --no-tags to update-branch display, removed TODO comments
README.md	Updated section header to "GIT CLONE OPTIONS" to match help text