feat(worktree): Add worktree commands, per-worktree trunks, and worktree filtering

[ed-irl]

Adds first-class support for working across multiple Git worktrees with independent trunks, so repo sync and restacks in different worktrees no longer contend on a single shared trunk checkout.

Commands

• gs worktree create <path> (alias wtc): creates a worktree with its own local trunk branch that tracks the same remote ref as the canonical trunk. --trunk names it (defaults to the directory name), --no-trunk keeps the legacy detached-HEAD behavior, and -b/--branch creates a tracked branch stacked on the worktree trunk.
• gs worktree list (alias wtls): lists worktrees with their checked-out branch and stack.

Per-worktree trunks

• New worktree-trunks registry in internal/spice/state: WorktreeTrunk, Register/Unregister/WorktreeTrunks, IsTrunk, and TrunkFor(path). The key is additive and optional, so existing repositories and older binaries are unaffected.
• BranchGraph now treats every registered worktree trunk as a graph root: downstack/Bottom traversals stop at any trunk, and StacksInWorktree returns complete stacks anchored in a given worktree.
• sync distinguishes the canonical remote trunk from the local trunk it pulls into for the current worktree; in the primary worktree the two are identical.

Worktree-scoped filtering

• -w/--worktree on log short (implies --all) filters the listing to stacks present in the current worktree and roots them under that worktree's trunk.
• -w/--worktree on repo restack limits restacking to stacks with a branch in the current worktree.

Adds Repository.WorktreeAdd, regenerated mocks, help/CLI-reference docs, a changelog entry, and unit + script tests covering trunk registry behavior, graph rooting, and the new commands and filters.

Part of #1241

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat(worktree): Add worktree commands, per-worktree trunks, and worktree filtering #1162 ◀
    • worktree: Add worktree management commands and per-worktree trunks #1163
      • feat: add worktree management commands, per-worktree trunks, and worktree-aware filtering #1164
        • worktree: Add management commands, per-worktree trunks, and worktree-aware filtering #1222
          • feat: add worktree management commands, per-worktree trunks, and worktree-aware filtering #1223
            • feat: add worktree management commands and per-worktree trunks #1224

_{Change managed by git-spice.}

[abhinav]

working on merging functionality from this stack next