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

[ed-irl]

Add 'gs worktree create' and 'gs worktree list'. By default 'worktree create' sets up a per-worktree trunk: a local branch tracking the same remote ref as the canonical trunk, so sync and restack in different worktrees never contend on a shared trunk checkout. --no-trunk preserves detached-HEAD behavior; -b stacks a tracked branch on the worktree trunk.

State gains a worktree-trunk registry (state/worktree.go) persisted under an additive, optional key. Store.IsTrunk now treats any registered worktree trunk as a graph root, and TrunkFor resolves the trunk for a given worktree. BranchGraph carries the full trunk set so traversals (Bottom, Downstack) stop at worktree trunks, and StacksInWorktree yields complete stacks touching a worktree.

Add '-w/--worktree' to 'log short' and 'repo restack' to scope output and restacking to the current worktree's stacks. Sync pulls the canonical remote trunk into the worktree's local trunk.

[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.}