feat: add `wt pr` command to create PR from worktree branch

[notdp]

Feature Request

Add a wt pr command that creates a pull request from the current worktree's branch to the main branch.

Proposed Usage

# From within a worktree
wt pr                    # Create PR to default base branch
wt pr --base develop     # Create PR to specific base branch
wt pr --draft            # Create as draft PR

Motivation

When working with worktrees, a common workflow is:

1. wt add <name> - create worktree
2. Make changes and commit
3. Create PR (currently requires manual gh pr create or web UI)
4. wt merge <name> / wt rm <name>

Adding wt pr would streamline step 3 and keep the workflow within wt.

Implementation Notes

• Could wrap gh pr create if available
• Auto-detect base branch from config or default branch
• Support common PR options (title, body, draft, reviewers)