doc: derive the commit author from the authenticated account

[albertodeluigi]

CLAUDE.md names a single author for every commit:

Author every commit as GracedEternalKingCabbageMan <151803062+...>.
Never a personal address.

An agent working for anybody else follows this literally and signs a colleague's
name to code that colleague never wrote or reviewed. Git does not verify the
author field — it is free text from the committer's config — so the commit lands
authored by the named person, linked to their account by the noreply address,
and counted in their contribution graph. The push is authenticated; the
authorship is not.

It has already happened once, in Sequentia, during a consensus fix that
reached the default branch. Correcting it took a history rewrite and a
force-push over commits other machines had already pulled.

Why not simply list the current committers

Because that is wrong for the same reason, only less often: more than two
accounts can push in this organisation, and any list is stale again the moment
somebody new arrives or one of them leaves. So derive the author from the
account the session is authenticated as:

git -c user.name="$(gh api user --jq '.name // .login')" \
    -c user.email="$(gh api user --jq '"\(.id)+\(.login)@users.noreply.github.com"')" \
    commit ...

gh api user carries the display name, and the id and login compose the GitHub
noreply address that links a commit to its account. Verified against every
human account that commits in this organisation. Nothing to maintain, and the
rule against private addresses is unchanged.

The change also says plainly what not to do, because the wrong answer is the
one nearest to hand: the author of the last commit is whoever pushed last, and
says nothing about who is working now.

Why this repository

CLAUDE.md opens by saying these rules are the same in every Sequentia
repository and are repeated in each one. That is accurate — 25 of the 27
non-archived repositories carry this identical line — and it is why the fix is
worth nothing in only one of them: an agent reads whichever copy it happens to
start from. This is that sweep; ConcatenaLabs/Sequentia#209 carries the same
edit.