Starter prompts refactored and moved

[gsuuon]

I've refactored and moved the starter prompts from require("llm.starter_prompts") to require("llm.prompts.starters"). The helpers have been moved out to their own files (like markdown extract, line cutoffs, openapi schema parsing) - I'll treat these helpers as part of the public API.

I've also added standard prompt types and provider-specific adapters to make it easier to setup cross-provider prompts. This makes it simpler to get a second opinion from a different provider while re-using the same prompt builder code. External plugin llm upgrading has been moved from 'llm.prompts' to 'llm.prompts.scopes'.

There are utilities for prompt composition -- check out the openapi starter to see an example of utilizing multiple prompts.