Add Copilot guidance on reusing existing library functions

[Copilot]

Adds technical instructions to .github/copilot-instructions.md emphasizing code reuse to prevent duplication of existing formulas and utilities.

Changes

New "Code Reuse and Avoiding Duplication" section

• Added as first subsection in Coding Guidelines for visibility (marked CRITICAL)
• Lists all reusable functions by category: temperature conversions, humidity calculations, pressure/altitude calculations, utilities, and constants
• Provides discovery methods: file search keywords, module indexes, similar formula imports
• Includes ❌ BAD vs ✅ GOOD examples:

// ❌ BAD - Reimplementing saturation vapor pressure
export function badHumidityCalc(temperature: number): number {
  const svp = 611.2 * Math.exp((17.62 * (temperature - 273.15)) / (243.12 + (temperature - 273.15)));
  // Don't reimplement existing formulas!
}

// ✅ GOOD - Reusing existing saturation vapor pressure
import { saturationVaporPressure } from './humidity';

export function goodHumidityCalc(temperature: number): number {
  const svp = saturationVaporPressure(temperature);
  // ... rest of calculation
}

Updated "Common Mistakes to Avoid"

• Elevated "Code Duplication" to Merge dev into main #1 priority
• Added explicit warning against reimplementing temperature conversions

Enhanced "New Formula Checklist"

• Added step 0: "Check for Existing Code" before implementation begins
• Added checklist item to verify reuse of existing library functions

Note

Function names use "Celcius" spelling (e.g., kelvinToCelcius()) matching the existing codebase API, not standard "Celsius" spelling.

Original prompt

Make a technical note for Copilot that it should use existing formulas and functions in the code libary as much as possible in new code, to avoid duplicates.

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