/review base-branch picker includes current branch (e.g. main -> main)

Summary

When using /review -> Review against a base branch, the picker includes the currently checked out branch as a base option.

Example shown in UI while on main:

• main -> main

Why this is a problem

Selecting the current branch as its own base is not a meaningful comparison target for PR-style review and is confusing in the picker.

Reproduction

1. Checkout main (or any branch).
2. Run /review in TUI.
3. Choose Review against a base branch.
4. Observe that the list contains <current> -> <current>.

Actual

Current branch is included in the base-branch options.

Expected

Current branch should be excluded from base-branch options (or clearly disabled as non-selectable).