feat(worktree): add worktree commands, per-worktree trunks, and worktree-aware filtering

[ed-irl]

Add gs worktree create and gs worktree list.

worktree create sets up a per-worktree trunk: a local branch (named
after the worktree directory, or set with --trunk) that tracks the same
remote ref as the canonical trunk. Stacks created in the worktree are
based on this branch, so repo sync and restacks in different worktrees
never contend on a single shared trunk checkout. Use -b to also create a
tracked branch on the worktree trunk, or --no-trunk for the legacy
detached-HEAD behavior.

The state store gains a per-worktree trunk registry (an additive,
optional key, so older repos and binaries are unaffected). Trunks are
resolved per-worktree via TrunkFor, and IsTrunk now treats every
registered worktree trunk as a graph root: traversals and the branch
graph stop at them, and they may not themselves be tracked or deleted.

Add -w/--worktree to log short and repo restack to scope output
and restacking to stacks with a branch checked out in the current
worktree, via BranchGraph.StacksInWorktree. The sync handler now
distinguishes the canonical remote trunk from the local worktree trunk
it pulls into, and restack gains WorktreeFilter and SkipCheckout.

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat(submodule): track and manage submodule branch associations #1178
    • feat(submodule): add git-layer primitives and transactional/recursive handlers #1179
      • submodule: associations follow checkout, inherit on create, merge on fold #1180
        • feat(worktree): add worktree commands, per-worktree trunks, and worktree filtering #1181
          • feat(worktree): add worktree commands, per-worktree trunks, and worktree-aware filtering #1182 ◀
            • feat(worktree): Add worktree commands, per-worktree trunks, and worktree-aware filtering #1183

_{Change managed by git-spice.}