feat: add worktree management commands and per-worktree trunks

[ed-irl]

Add gs worktree create and gs worktree list for managing Git worktrees.

worktree create sets up a per-worktree trunk by default: a local branch
(named after the worktree directory, or set with --trunk) that tracks the
same remote ref as the canonical trunk. Stacks in different worktrees are
based on their own trunk, so repo sync and restacks never contend on a
single shared trunk checkout. --no-trunk keeps the detached-HEAD behavior,
and -b creates a tracked branch stacked on the worktree trunk.

Worktree trunks are recorded in a new, additive state key and treated as
graph roots: IsTrunk and BranchGraph traversals stop at any trunk, and
TrunkFor resolves the trunk that applies to a given worktree. Trunk sync
now pulls the canonical remote ref into the worktree's local trunk.

Add -w/--worktree to log short and repo restack to scope operations to
stacks with a branch checked out in the current worktree.

Part of #1247

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat: add worktree management commands and per-worktree trunks #1187 ◀
    • anchor: Make restack and log anchor-aware #1302
      • anchor rm: remove an anchor worktree and dissolve its anchor #1303
        • repo: Add exclusive mode (park/restore/exclusive) #1304
          • docs: Add parallel worktrees guide #1305

_{Change managed by git-spice.}