upgrade

Upgrading is diff-and-decide, not copy-over. The toolkit records what a
repository was initialized from, and that recorded commit is the merge base
that tells a customized file from an untouched one.

Find the standard

repo/upgrade.md in a dc-agentics checkout. Operation 2 needs both
repositories present, because it diffs two commits of the toolkit; ask for the
checkout path rather than guessing it. Read the file and follow it - the
per-path merge policy and the diff mechanics are both there, and neither is
guessable.

Choose the operation from $ARGUMENTS

Operation 3 needs no toolkit checkout; run it from inside the repository
being converted. Run from a toolkit checkout instead, it first checks that the
two agree, and declines rather than mixing a payload delta into a layout change.

Then

The tree being changed must be clean - git status --porcelain empty,
untracked included - or decline and say so; the document says why, and what
the one-command undo is that a clean start makes possible.

Produce the plan first: what changed between the two commits, what it would do
to each file, and what needs a decision. Apply as a second step, and let
source-control.mode in the target's .dc-agentics.yaml decide what may be
committed without asking.

Operations 2 and 3 finish with dotnet build, dotnet test, and /scrub -
the scrub checks are the post-upgrade checks. Operation 1 has nothing to build
in the toolkit; the document says how a bump is proven.