Both skills said to derive the main clone from the checkout without saying
how, and dsh-upgrade names dsh-customize as the owner of checkout discovery
— so the technique belonged there and was missing.
dsh-customize now gives it: `git rev-parse --git-common-dir` from the
checkout yields the shared git directory, whose parent is the main clone.
It also names the two ways to get this wrong — the answer is relative for a
plain clone, and paths must be compared physically, since macOS reaches
/var through a symlink to /private/var.
dsh-upgrade links to that procedure rather than restating it.
Verified against both shapes: an adopted clone outside the container, and a
curl-shaped install whose clone is at <source>/master.
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