feat: add worktree management 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 dir, or --trunk) that tracks the same remote ref as the canonical trunk, so sync and restack in different worktrees never contend on a single shared trunk checkout. --no-trunk keeps the legacy detached-HEAD behavior; -b also creates a tracked branch stacked on the worktree trunk.

Persist the per-worktree trunk registry in the state store under an additive, optional worktree-trunks key (older binaries and pre-feature repos see no entry and fall back to the canonical trunk). Add Store.IsTrunk, TrunkFor, WorktreeTrunks, and register/unregister methods. Trunk checks throughout the state and branch-graph layers now treat every registered worktree trunk as a graph root, so traversals (Downstack, Bottom, tx path-finding) stop at them and a branch may be stacked directly on a worktree trunk.

Add -w/--worktree to log short and repo restack to scope operations to stacks with at least one branch checked out in the current worktree, backed by BranchGraph.StacksInWorktree. sync pulls the canonical remote trunk into the worktree's local trunk. repo restack gains SkipCheckout and handles checkout itself.

Includes unit tests for the trunk registry and graph roots, plus test scripts covering worktree create/list, the -w filters, and branch create/restack when trunk is checked out elsewhere.

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat(worktree): Add worktree commands, per-worktree trunks, and worktree filtering #1162
    • worktree: Add worktree management commands and per-worktree trunks #1163
      • feat: add worktree management commands, per-worktree trunks, and worktree-aware filtering #1164
        • worktree: Add management commands, per-worktree trunks, and worktree-aware filtering #1222
          • feat: add worktree management commands, per-worktree trunks, and worktree-aware filtering #1223 ◀
            • feat: add worktree management commands and per-worktree trunks #1224

_{Change managed by git-spice.}