One in-process builtin (no network, runs in executorβs own process).
call.contract_idβs last segment names the operation β
e.g. robonix/system/executor/builtin/read_file β read_file.
Resolve a user-supplied path and ensure it stays within the workspace root.
Returns the canonical path on success, or an error if the path escapes the
allowed directory (path traversal).