Assistant: Rethink `session_type`

When creating a chat session in the assistant, we assign that session a type - either workflow_template or job_code

Back when we had split assistants, this made sense. But now the session is more fluid. You can start a session in workflow chat, then move into job code, then go back and edit your workflow.

This is affecting our reporting and monitoring. When we filter messages by which chat service is being used, the session is usually (or maybe always?) workflow_template, because that's where we start. So it's hard now to filter for all job_chat sessions.

This problem will get worse as we move to the global assistant. In a few months time (hopefully) Lightning won't know which agent its using, so we can't make this filter at all.

So I'm not sure of the answer here. Certainly I think we need to drop the type from the session, it's very confusing. Should we add an endpoint or agent key to each message? Maybe we should add the page URL - or a simplified one - to each message?

No files, tests, or entry points are named in the issue. Start by tracing assistant session creation, message recording, and the reporting or monitoring filters; the work is done when the team agrees on a replacement for session_type and the chosen model supports reliable filtering across workflow, job-code, and future global-assistant sessions.