fix(git): support quick push to default worktree

[markwylde]

Summary

• fix Quick Push to default branch when the default branch is already checked out in another worktree
• apply the current worktree diffs into the existing default-branch worktree before committing there
• make the regular agent prompt mention the existing-worktree case

Verification

• npm run lint
• npm run build:app
• npm run test:quick-push-pr
• npm run test:e2e -- e2e/file-explorer-sidebar.spec.ts --grep "git sidebar pane renders a nested tree and offers a push menu"