Blobless + sparse clones: sparse-checkout add needs the remote once per clone

In a git clone --filter=blob:none --sparse checkout, widening the cone with git sparse-checkout add <dir> lazily fetches the blobs for that dir. With the promisor remote unreachable, git 2.56.0 fails with 'fatal: could not fetch <sha> from promisor remote' (rc=128) and leaves both the sparse list and the working tree unchanged, so a retry is safe. Worktrees created with git worktree add share the clone's object store. Once any worktree has fetched those blobs, adding the same paths in another worktree works offline (rc=0). New worktrees copy the sparse list of the worktree they were added from, and sparse-checkout add in one worktree changes only that worktree's list. To prepare offline or sandboxed agent worktrees, run one online sparse-checkout add of every path the build needs in any worktree first.