CLI — one install, one usage page

The CLI is the same Core the agent and the IDE already call.

• Install is one page with tabs for CLI, IDE, and MCP. There is no separate CLI install lane in the sidebar.
• Usage is the same shape: pick the surface you are in.
• prism doctor still prints the folder it chose and the index state. A warn on a first run is expected.

Analysis does not leave the machine. See Quickstart for the first five minutes.