feat(email): Add support for email body description prefix

[mchehab]

There have been discussions about having custom per-subsystem body prefixes to add instructions about how to use Sashiko's review:

• at the media subsystem, there was a proposition to add a prefix to warn new developers like this:

    This is an LLM-generated review that can be prone to hallucinations.
    It is fine to ignore this review.
    When in doubt about some of the findings, please consult an experienced developer.
  

• recently, at Ksummit ML, about adding a custom per-subsystem note about what it is expected with regards to pre-existing conditions:
  https://lore.kernel.org/all/efdce5c5-13a1-4dec-9823-bb2a37084538@roeck-us.net/

Add an option to optionally set it at the sashiko.dev email policy.

NOTE: compile-tested only.