Improve cost segregation for different tasks within a workspace

Re https://github.com/coder/mux/pull/1658#issuecomment-3775922643 and https://news.ycombinator.com/item?id=46695415#46699650

Currently, Mux workspace costs are presented as an entire sum of costs all requests in that workspace, with no consideration for specific tasks, PRs, or commits. This leads to:

- Misleading costs on external artifacts such as PRs
- A conflicting incentive for long-lived workspaces:
- From a context perspective, it's often more efficient to compact and continue with an existing workspace to work on related tasks
- From a cost reporting perspective, it's more efficient to create fresh workspaces often

Idea: feed cumulative and last-request costs as synthetic messages to the LLM, perhaps the LLM will generally be smart enough to know to only include costs from message B to message C if message A had little to do with a PR.