Quite a few changes: 0.24.2...main

A notable new feature is:

• Allow fragment loader plugins to return a mixture of fragments and attachments #972

Which will be first demonstrated by this plugin: https://github.com/simonw/llm-video-frames

First need to land:

• Fixes --continue bug and adds --database argument to llm chat #939
• llm/default_plugins: add o3 model #945
• Add o4-mini #976
• And write the release notes