feat(worktree): add worktree management commands and per-worktree trunks

[ed-irl]

Adds gs worktree create and gs worktree list, plus worktree-aware filtering across the log and restack commands.

Per-worktree trunks

gs worktree create <path> sets up a new worktree with its own local trunk branch (named after the directory, or via --trunk) that tracks the same remote ref as the canonical trunk. Stacks created in a worktree base on this branch, so gs repo sync and restacks in different worktrees no longer contend on a single shared trunk checkout. -b/--branch also creates a tracked branch stacked on the worktree trunk; --no-trunk keeps the legacy detached-HEAD behavior.

The registry lives in a new, additive worktree-trunks state key — older binaries and pre-existing repos simply see no entry. Store.IsTrunk/TrunkFor treat registered worktree trunks as trunk-equivalents, and BranchGraph traversals (Bottom, Downstack, etc.) now stop at any trunk, not just the canonical one.

Worktree-scoped filtering

-w/--worktree on log short/log long and repo restack limits output to stacks with a branch checked out in the current worktree, via the new BranchGraph.StacksInWorktree. repo restack gains an internal SkipCheckout so it can filter without forcing a checkout.

Test plan

• Store unit tests: registry round-trip, reopen persistence, trunk-as-graph-root, canonical-trunk rejection
• BranchGraph unit test: worktree trunk terminates traversal
• Test scripts: worktree create/list, log short -w and repo restack -w filtering, branch create/restack when trunk lives in another worktree
• mise run generate for help/docs

[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 ◀

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