Adds gs worktree create and gs worktree list, plus worktree-aware filtering across the stack.

Commands

• gs worktree create <path> (alias wt c): creates a Git worktree. By default it sets up a per-worktree trunk — a local branch (named after the worktree dir, or --trunk) that tracks the same remote ref as the canonical trunk, so sync/restack in different worktrees never contend on one shared trunk checkout. --no-trunk keeps the legacy detached-HEAD behavior; -b/--branch also creates a tracked branch stacked on the worktree trunk.
• gs worktree list (alias wt ls): lists worktrees with their checked-out branch and stack.

Per-worktree trunks

• New worktree-trunks state key registers worktree-trunk branches. The key is additive and optional, so older repos/binaries are unaffected.
• Store.IsTrunk/TrunkFor/WorktreeTrunks treat registered branches as trunk-equivalents: valid stack roots that may not themselves be tracked or deleted.
• BranchGraph stops traversals at any trunk-equivalent, and gains StacksInWorktree to collect full stacks touching a worktree.
• sync pulls the canonical remote trunk into the worktree's local trunk; the split is a no-op in the primary worktree.

Worktree filtering

• -w/--worktree on log short and repo restack scopes operations to stacks checked out in the current worktree.
• repo restack gains a WorktreeFilter and SkipCheckout on the restack handler.

Tests

• Unit tests for the worktree-trunk registry and graph rooting.
• Test scripts: worktree_create, worktree_list, log_short_worktree_filter, repo_restack_worktree_filter, and worktree-trunk branch create/restack regressions.