feat: add worktree management with per-worktree trunks

[ed-irl]

Adds gs worktree create and gs worktree list, plus the per-worktree trunk machinery they rely on, and worktree-aware filtering for log and repo restack.

Per-worktree trunks:

• A worktree may register a local trunk branch 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.
• state.Store persists the registry under a new optional
  worktree-trunks key (missing key = no worktree trunks, so old
  repos and binaries are unaffected). New Store methods: IsTrunk,
  TrunkFor, WorktreeTrunks, Register/UnregisterWorktreeTrunk.
• Trunk checks throughout state and the branch graph go through
  IsTrunk / isTrunk, so every registered worktree trunk is a graph
  root: traversals (Bottom, Downstack) stop at it and it can be a
  base but not itself tracked.

Commands:

• gs worktree create <path> creates a Git worktree and, by default,
  a per-worktree trunk named after the directory (or --trunk),
  tracking the canonical trunk's remote ref. -b stacks a tracked
  branch on it; --no-trunk keeps the legacy detached-HEAD behavior.
• gs worktree list shows each worktree, its checked-out branch, and
  its stack.
• git.Repository gains WorktreeAdd.

Worktree-aware filtering:

• BranchGraph.StacksInWorktree yields the full stacks that have any
  branch checked out in a given worktree.
• log short -w/--worktree filters the listing to the current
  worktree's stacks; repo restack -w/--worktree restacks only those
  stacks. List and sync render and pull against the worktree's own
  trunk via TrunkFor.

Tests:

• Unit tests for the worktree-trunk registry (persistence, reopen,
  graph-root semantics, canonical-trunk rejection) and for
  worktree-trunk traversal in the branch graph.
• Script tests: worktree create/list, log/repo-restack -w filtering,
  and branch create/restack when trunk is checked out elsewhere.

[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: add worktree management with per-worktree trunks #1250 ◀

_{Change managed by git-spice.}