Hold the agent prompt and tools in one place

Hold the agent prompt and tools in one place

The system prompt of the UniiChat design and the zoom and date tools
live in prompt.go, so the turns and the compactions send the same bytes
and share one prompt cache. The prompt is the spec text with the agent's
name in place of Unii and the kind reply in place of unii. It holds no
date and no state.

The prompt describes only what OneChat can do, so it leaves out the spec
text on images, the name form of zoom, subagents, background tasks, the
user's computers, and the user's instructions. Each part returns with
the issue that adds its feature. The tool schemas are strict, with every
property required, and encode to the same bytes on each call.

Issue: b14d537