PR base branch default should respect worktree base, not repo default

The "Create PR" button in the Claude Code UI defaults the base branch to the repository's default branch (e.g. master), regardless of which branch the current worktree was created from.

In workflows where feature work is done off a long-lived integration branch (e.g. feature/<something>) rather than master, this is misleading: the button shows a diff of ~110k lines (the full integration branch vs master) and would create a PR targeting the wrong base if clicked.

Expected: The button should default to the branch the current worktree/branch was created from (the merge-base or upstream tracking branch), not the repo's default branch.

Workaround: Use gh pr create --base <branch> from the terminal instead.