worktree: Add worktree 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: a local branch (named after the worktree directory, or set with --trunk) that tracks the same remote ref as the canonical trunk, so sync and restack in different worktrees never contend on a single shared trunk checkout. --no-trunk preserves the old detached-HEAD behavior, and -b stacks a new tracked branch on the worktree trunk.

Introduce a persisted per-worktree trunk registry in state.Store (worktree.go): RegisterWorktreeTrunk, UnregisterWorktreeTrunk, WorktreeTrunks, TrunkFor, and IsTrunk. The registry lives under an additive, optional store key, so older repos and binaries simply see no entry. IsTrunk now treats both the canonical trunk and any registered worktree trunk as graph roots; BranchTx upsert/delete/path checks switch from == trunk to IsTrunk.

Teach BranchGraph about trunk-equivalents: it loads WorktreeTrunks from the BranchLoader, treats all of them as roots in Downstack/Bottom, and adds StacksInWorktree to enumerate complete stacks that have at least one branch checked out in a given worktree.

Wire worktree-aware behavior through the handlers:

• list: add -w/--worktree to log short/log long, scoping output to the current worktree's stacks and rendering them under the worktree's own trunk (TrunkFor).
• restack: add WorktreeFilter and SkipCheckout to the request; repo restack -w limits restacking to stacks in the current worktree.
• sync: split canonical trunk (remote ref to pull from) from the worktree's local trunk (the branch to pull into), a no-op in the primary worktree.

Add git.Repository.WorktreeAdd, expose GitWorktree.RootDir on spice.Service, and add spice.Service.WorktreeTrunks/TrunkFor. Includes unit tests, test scripts, regenerated docs/help, and a changelog entry.

[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.}