fix(checkpoint): stamp on detached ctx and match remote URL exactly

Address two gaps in the checkpoint-remote stamping:

• A filtered fetch that times out leaves its context past the deadline, so the post-fetch git-config read/write no-op'd and the phantom remote git had already recorded was never stamped — reintroducing the same permanent miss this stamping prevents. Run the stamp helpers on a context detached from the fetch deadline (context.WithoutCancel plus a short bound so a stuck config lock can't hang the caller).

• gitRemoteSectionExists matched on a "remote.<url>." string prefix, so a longer URL sharing a prefix (e.g. a ".../repo.git" section) made a ".../repo" fetch look pre-existing and skipped its stamp. Compare the full URL subsection exactly instead.