Worktree management commands and worktree-aware filtering

## Problem

Working across multiple Git worktrees is awkward with git-spice.
Stack-wide operations act on all tracked branches regardless of which
worktree they belong to, so running a command in one worktree can churn
stacks that are checked out elsewhere. There are also no first-class
commands for creating or listing worktrees.

## Proposal

Add worktree management and worktree-aware filtering:

- `gs worktree create` and `gs worktree list` commands for managing Git
worktrees.
- A `-w/--worktree` flag on `log short` and `repo restack` that limits
the operation to branches belonging to stacks checked out in the
current worktree.

This lets a user keep multiple worktrees in play without operations in
one affecting unrelated stacks in another.

## Tracked by

- #1187 — worktree management commands and worktree-aware filtering