fix(review): exclude current branch from base branch picker

[middleDuckAi]

Summary

• exclude the currently checked out branch from the review base-branch picker
• mirror the fix in both codex-tui and codex-tui-app-server
• add deterministic regression tests with a temporary git repo fixture

Why

• the picker currently includes the active branch, which is not a valid base-branch choice for the review flow

Testing

• cargo test -p codex-tui review_branch_picker_excludes_current_branch
• cargo test -p codex-tui-app-server review_branch_picker_excludes_current_branch
• just fix -p codex-tui
• just fix -p codex-tui-app-server
• CARGO_TARGET_DIR=/tmp/codex-target-12942-tui-full cargo test -p codex-tui
• CARGO_TARGET_DIR=/tmp/codex-target-12942-app-full cargo test -p codex-tui-app-server