Style guide

The style guide covers language, theme and user interface (DEV-07). It exists before the first webview or graphical feature, and pull-request review checks conformance to it (DEV-10).

Section	North Star	Status
Language	Say what an analyst would say: plain British English, glossary terms, never software jargon.	Version 0
Theme	Look native to VS Code; keep colour for what the data means.	Skeleton; filled from Phase 1
User interface	Look native to VS Code; keep colour for what the data means.	Skeleton; filled from Phase 1