Built-in tools

Always in the catalog, called as builtin__<name>.
infer and decide need a configured model: a chat model for infer, a decision model for decide. reshape makes no model call.
To replace one, put a tool file with the same name in <tools dir>/builtin/, such as .graph/tools/builtin/reshape.yaml.

Name	Description
`builtin__infer`	Runs one model call over an instruction and returns text or structured JSON.
`builtin__decide`	Asks a decision model typed questions about one piece of data.
`builtin__reshape`	Builds a value in a new JSON shape from earlier step results.