Warn what changing the model, thinking or effort will cost before it is sent

[shellicar]

Summary

• The status bar names what changing the model, thinking or effort would re-write, and what that costs.
• The size and price of a model change are counted by the model being switched to.
• A resumed conversation comes up on the model, thinking and effort it last ran under.
• Every audit line records the thinking and effort its turn was sent with.
• .claude/request-composition.md records how a request is assembled and what invalidates its cache.