SHA-256 a byte slice and return it as the prefixed digest string
"sha256:<lowercase-hex>". Used by difflore-cli’s MCP install manifest
(item ⑤b) to hash the exact DiffLore config block we render, so a later
agents update can tell “unchanged since DiffLore wrote it” (safe to
upgrade) from “the human edited it” (must not clobber). Keeping the hashing
here lets the CLI reuse the workspace sha2 dep instead of pulling in a
second hashing crate, and pins the algorithm choice to one place. The helper
is pure — it never reads files, the keyring, or any repo-scoped state.