worktree: Add worktree management commands, per-worktree trunks, and worktree-aware filtering

[ed-irl]

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

Commands:

• worktree create <path> creates a Git worktree. By default it sets up a per-worktree trunk: a local branch (named after the worktree directory, or --trunk) that tracks the same remote ref as the canonical trunk. -b/--branch also creates a tracked branch stacked on that trunk; --no-trunk preserves the legacy detached-HEAD behavior.
• worktree list lists worktrees with their checked-out branch and stack.

State: a new optional, additive worktree-trunks registry maps each per-worktree trunk branch to its owning worktree root. Store gains IsTrunk, TrunkFor, WorktreeTrunks, and Register/UnregisterWorktreeTrunk. Branch-tracking invariants now treat any registered trunk as a graph root: trunks may not be tracked or deleted, and branches may stack directly on them.

BranchGraph treats all trunk-equivalents as roots, so downstack and Bottom traversals stop at a worktree trunk instead of bleeding into the canonical trunk. Adds StacksInWorktree to collect full stacks with a branch checked out in a given worktree.

Filtering: log short -w/--worktree and repo restack -w/--worktree scope output and restacking to stacks present in the current worktree. The sync handler now distinguishes the canonical remote trunk from the local per-worktree trunk it pulls into, fast-forwarding the local trunk from the remote ref.

Part of #1244

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat: Add worktree commands with per-worktree trunks #1249
    • feat(worktree): Add worktree commands, per-worktree trunks, and worktree-aware filtering #1188
      • worktree: Add worktree commands, per-worktree trunks, and worktree-aware filtering #1165
        • feat: Add worktree commands, per-worktree trunks, and worktree filters #1172
          • worktree: Add worktree management commands, per-worktree trunks, and worktree-aware filtering #1173 ◀
            • integration: Wire auto-rebuild hook on restack commands #1174
              • integration: Auto-rebuild integration branch after restack #1175
                • integration: Auto-rebuild integration branch on restack #1176
                  • integration: Add AI-driven conflict resolution #1251
                  • integration: Move regeneration out of merge driver #1252
                  • integration: accept-incoming fallback after auto-resolve #1253
                  • integration: submit tip downstacks alongside the integration branch #1254
                  • integration: surface integration branch and tips in gs ll #1255
                  • integration: short-circuit gs branch checkout for the integration #1256
                  • integration: gs ss from integration submits all tip stacks #1257
                  • integration: tip clean prunes subsumed tips #1258
                  • integration: tip advance moves tips to upstack leaves #1259
                  • integration: rebuild fixes and reliability improvements #1260

_{Change managed by git-spice.}