Worktree management commands and worktree-aware filtering

[ed-irl-codebot]

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

• feat: add worktree management commands and per-worktree trunks #1187 — worktree management commands and worktree-aware filtering