worktree: Add 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 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 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 stacked 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 repositories and binaries are unaffected).
Trunk-equivalence now flows through the branch graph and branch
transactions: any registered worktree trunk is treated as a graph
root, so traversals stop at it and it cannot itself be tracked.

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, and resolve the displayed/synced trunk per worktree
via Store.TrunkFor.

Includes unit tests for the registry and graph behavior, plus test
scripts covering worktree create/list, worktree-aware filtering, and
branch create/restack when the 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.}