Worktree branches should not set origin/master as upstream

When using claude -w (worktree mode), the created branch sets origin/master as its upstream. This is problematic for the common workflow where pushing directly to master is not allowed (branch protection) and a PR is required.

Current behavior:

• New worktree branch is created tracking origin/master
• git status shows misleading ahead/behind counts relative to master
• git push without arguments fails or targets the wrong branch

Expected behavior:

• Create the branch with no upstream set
• Let the upstream be set naturally on first push via git push -u origin HEAD

This is the safest default for repos with branch protection, where the workflow is always: create branch → push → open PR. Having no upstream avoids confusion and prevents accidental pushes to the wrong target.