feat: Add worktree commands, per-worktree trunks, and worktree filters

[ed-irl]

Add 'gs worktree create' and 'gs worktree list' commands. By default,
'worktree create' sets up a per-worktree trunk: a local branch (named
after the worktree directory, or set with --trunk) that tracks the same
remote trunk as the main checkout. Stacks created in the worktree are
based on this branch, so 'gs repo sync' and restacks in different
worktrees never contend on a single shared trunk checkout. --no-trunk
keeps the legacy detached-HEAD behavior, and -b/--branch creates a
tracked branch stacked on the new trunk.

The state store gains a per-worktree trunk registry (WorktreeTrunk,
Register/Unregister, TrunkFor, IsTrunk). Trunk checks throughout the
store and branch graph now recognize any registered worktree trunk as a
trunk-equivalent graph root, so traversals stop at it and branches may
be stacked directly on it.

'log short' and 'repo restack' gain -w/--worktree to scope output and
restacking to stacks with a branch checked out in the current worktree,
backed by BranchGraph.StacksInWorktree. 'repo restack' also skips its
trailing checkout. SyncTrunk now distinguishes the canonical remote
trunk from the local trunk it pulls into, fetching the remote ref into
the worktree's own trunk.

Adds Repository.WorktreeAdd for creating worktrees.

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.}