bridge: text transcript

[progrium]

We need the backend to generate a text transcript of the session for use with LLMs. This is similar to how the frontend generates a UI timeline/history of events, but this is done on the backend and has a couple of specific requirements:

• not structured data, its text (though could have a structured intermediate representation if needed)
• needs to have a definable "window size" that can be adjusted based on the LLM context size (usually in tokens, but could be approximated in characters)
• needs to filter out (or have a statement indicating it was redacted) other assistant responses

We would use this as the input prompt for LLM assistant queries, though for now I think we should just send the immediate query for function choosing ("bridge, please add 10 and 20").