`.lanes` was spelled twice — the default and the help string that describes it —
which is the shape that reads as correct forever and is wrong the first time
either moves. One declaration now, and the help asks for it.

It is also a profile setting: `orchestrate on lanes at=../lanes`. A project whose
worktrees must sit outside the repository has a real reason — a watcher that
scans it, a tool that walks every subdirectory — and only it knows. A relative
path resolves against the ROOT, never the cwd, so a lane opened from a
subdirectory does not land in it.

Discovery is unaffected and always was: `Checkout::lanesOf` asks git for its
worktrees, so a lane put anywhere is still found by `lane list` and `upgrade`.
The location is an argument, never an assumption.