assistant: Make scripting a first-class concept instead of a tool

[agu-z]

This PR makes refactors the scripting functionality to be a first-class concept of the assistant instead of a generic tool, which will allow us to build a more customized experience.

• The tool prompt has been slightly tweaked and is now included as a system message in all conversations. I'm getting decent results, but now that it isn't in the tools framework, it will probably require more refining.

• The model will now include an <eval ...> tag at the end of the message with the script. We parse this tag incrementally as it streams in, so that we can indicate that we are generating a script before we see the closing </eval> tag. Later, this will help us interpret the script as it arrives also.

• Threads now hold a ScriptSession entity which manages the state of all scripts (from parsing to exited) in a centralized way, and will later collect all script operations so they can be displayed in the UI.

• script_tool has been renamed to assistant_scripting

• Script source now opens in a regular read-only buffer

Note: We still need to handle persistence properly

Release Notes:

• N/A