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

[ed-irl]

Adds a gs worktree command group and the per-worktree trunk machinery that lets stacks in different worktrees sync and restack without contending on a single shared trunk checkout.

Commands

• gs worktree create <path> (alias wtc): creates a worktree with its own local trunk branch that tracks the same remote ref as the canonical trunk. The trunk is named after the worktree directory or set with --trunk. -b/--branch also creates a tracked branch stacked on the worktree trunk; --no-trunk keeps the legacy detached-HEAD behavior.
• gs worktree list (alias wtls): lists worktrees with their checked-out branch and stack.

Worktree-aware filtering

• gs log short/log long gain -w/--worktree to show only stacks with a branch checked out in the current worktree (implies --all).
• gs repo restack gains -w/--worktree to restack only those stacks. The restack handler grows a WorktreeFilter and a SkipCheckout option.

State and graph

• New per-worktree trunk registry in the state store (worktree.go): persisted under an additive, optional worktree-trunks key so older binaries and pre-feature repos are unaffected. Adds RegisterWorktreeTrunk/UnregisterWorktreeTrunk, TrunkFor, WorktreeTrunks, and an IsTrunk check used by the branch transaction.
• BranchGraph treats every registered worktree trunk as a graph root, so downstack and bottom traversals stop there.
• BranchGraph.StacksInWorktree returns the full stacks touching a given worktree, backing both filters.
• git.Repository.WorktreeAdd wraps git worktree add; spice.GitWorktree exposes RootDir.

Sync resolves the local trunk to pull into via TrunkFor while still fetching from the canonical remote ref, so a linked worktree updates its own trunk.

[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.}