+idempotent-file-ops.added.md

Added SHA-1 idempotency primitives for CoreFileObject nodes:

• InfrahubNode.matches_local_checksum(source) / sync variant — compare a local bytes | Path | BinaryIO source against the node's server-stored checksum without invoking a transfer.
• InfrahubNode.upload_if_changed(source, name=None) / sync variant — stage + save only when the local source differs from the server, returning an UploadResult(uploaded, checksum) dataclass.
• download_file(..., skip_if_unchanged=True) — short-circuit the download when dest already exists on disk with a matching SHA-1. Returns 0 bytes written when skipped.

A shared sha1_of_source helper (streaming, 64 KiB chunks) centralises the hashing convention in infrahub_sdk.file_handler.