Add template support to llmtool decorator

[ncoop57]

@llmtool now accepts **tmpls kws to format docstrings, enabling reuse of common documentation text.

common_doc = "Some repeated text..."

@llmtool(foo=common_doc)
def my_func():
    "Unique part. {foo}"

Backwards compatible — @llmtool without args works as before.

[jph00]

Clever! :D