agent thread: two types of instructions

This feature is following our QA person suggestion (#28):
Instead of having one standard text displayed to the agent, the script will have two kinds of texts:

1. Instructions and guidance for the agent (`say...`, like we have today).
2. Quoted text suggestion to tell the user.

* [ ] design
* [ ] script representation
* [ ] script editor integration

The issue names no files, tests, or entry points. Start by examining the existing agent script and editor integration, then define the design and representation for separate agent guidance and quoted user text. Done means the design, script representation, and editor integration are all addressed.

Written by the indexing model from the issue text.