worktree: Add worktree management commands and per-worktree trunks

[ed-irl]

Adds a gs worktree command family and per-worktree trunk support:

• worktree create <path>: creates a Git worktree that, by default, gets
  its own trunk branch (named after the directory, or --trunk) tracking
  the same remote trunk as the main checkout. Use -b to also create a
  tracked branch stacked on it, or --no-trunk for detached HEAD at trunk.
• worktree list (ls): lists worktrees with their checked-out branch
  and stack.

Per-worktree trunks are stored in a new, additive worktree-trunks state
key, so older repos and binaries are unaffected. They are treated as
trunk-equivalents: each tracks the same remote ref as the canonical
trunk and acts as a graph root, so sync and restack in different
worktrees never contend on a single shared trunk checkout. repo sync
now pulls the remote canonical trunk into the worktree's local trunk.

Also adds worktree-aware filtering:

• log short -w/--worktree: shows only stacks with a branch in the
  current worktree.
• repo restack -w/--worktree: restacks only those stacks.

The branch graph and store gain WorktreeTrunks/TrunkFor/IsTrunk
helpers and a StacksInWorktree traversal to back these.

[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.}