job_chat: add more stylistic job writing instructions

[hanna-paasivirta]

Joe's observations from an evaluation session:

• OpenFN Block Usage: OpenFN blocks should be treated as a last resort for transformation. The goal should be to do as much transformation as possible within a single top-level operation.
• State Operator: Use the Lazy State Operator ($) instead of directly manipulating the state object.
• Defaulting Values: Be cautious or careful about defaulting values within the data structure.

Some of these might be in the docs (see state operator), but may need to be referenced in a prompt. A solution might look at fetching more relevant parts of the general docs, or add a job code style agent that can proofread generated code according to more style pointers and rules to avoid bloating the main agent.