fix(checkpoint): stamp skipFetchAll only on remotes we create

PR #1733 stamped skipFetchAll/skipDefaultUpdate onto any URL-keyed promisor section it found on every filtered fetch, rewriting config for remotes it did not create. Replace that with stamp-on-creation: check whether the remote.<url> section exists before the fetch and only stamp the skip flags when this fetch is the one creating it. Drop the promisor read entirely. Pre-existing remotes are left untouched so we never rewrite the user's git config, and adhoc checkpoint URL remotes still stay out of git fetch --all / git remote update.