docs: extend CONTEXT.md with Omarchy paths and WLED/sentinel glossary #55

Per readability review: add short glossary entries to CONTEXT.md for Omarchy paths (colors.toml, theme.name, background symlink), WLED concepts (seg.i spatial gradient, bri), and watcher vs Sentinel polling so code can stay terse without losing new readers. Only if CONTEXT.md already exists and project wants narrative docs.