feat(copy): add --work-dir option and extract shared repo_utils

[EspenAlbert]

Summary

Add --work-dir option to path-sync copy, matching the existing dep-update behavior. When set, destination repos are cloned into {work-dir}/{dest.name} instead of resolving from dest_path_relative relative to src_root.

Extract shared repo-resolution logic (resolve_repo_path, ensure_repo) into repo_utils.py so both commands use the same code path.

Reviewer notes

• ensure_repo only ensures existence: it clones missing repos, prompts to re-clone invalid ones, and returns the Repo for existing valid repos. It does not fetch or reset. The fetch-and-reset responsibility stays with prepare_copy_branch(from_default=True), which dep-update always uses and copy uses only when --checkout-from-default is set.
• ensure_repo behavior change for dep-update: previously _ensure_repo raised typer.Abort() on user decline; now raises ValueError. This avoids the typer dependency in the shared module.
• dest_path_relative remains required on Destination: when --work-dir is set, the field is ignored. Making it optional (defaulting to "") could be a follow-up if configs want to drop it entirely.