[Nigh]

Summary

• add CodeMirror floating autocomplete for earlier variables, custom functions, and registered built-ins
• accept active completions with Tab without changing normal Tab behavior
• theme the popup with equal edge padding and show concise built-in function descriptions
• add unit and Playwright coverage and update AGENTS.md

Verification

• npm test
• npm run check
• npm run build
• Playwright: 8 passed