feat: Add worktree commands with per-worktree trunks

[ed-irl]

Add gs worktree create and gs worktree list, backed by a per-worktree trunk registry, plus worktree-aware filtering for log and repo restack.

Per-worktree trunks:

• state: New worktree-trunks store key (additive/optional, so older
  repos and binaries are unaffected) registering local branches that
  are trunk-equivalents — each tracks the same remote ref as the
  canonical trunk. Store gains IsTrunk, TrunkFor, WorktreeTrunks, and
  Register/UnregisterWorktreeTrunk; branch transactions now treat any
  registered trunk as a graph root.
• spice: BranchGraph tracks the full set of trunk-equivalents so
  Downstack, Bottom, and Stack traversals stop at any trunk. Adds
  StacksInWorktree to enumerate stacks touching a given worktree.

Commands:

• worktree create <path>: by default creates a local trunk branch
  (named after the directory, or --trunk) tracking the remote trunk,
  so sync and restack in different worktrees never contend on one
  shared checkout. --no-trunk keeps detached-HEAD behavior; -b stacks
  a tracked branch on the worktree trunk.
• worktree list: shows each worktree, its branch, and stack.

Worktree-aware filtering:

• log short -w and repo restack -w scope to stacks with a branch
  checked out in the current worktree; -w implies --all for log.
• sync resolves the trunk to pull into via TrunkFor(worktree root),
  fast-forwarding the local trunk from the canonical remote ref.

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
      • feat(worktree): add worktree management commands and per-worktree trunks #1170
      • 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
      • feat: add worktree management with per-worktree trunks #1250

_{Change managed by git-spice.}