docs: add code examples for lib/money.ts integer-cents calculations

🎯 What needs to be done

Add clear, copy-pasteable TypeScript code examples in docs/DEVELOPMENT.md showing how to use PACT's integer-cents financial arithmetic utilities from src/lib/money.ts.

Why this matters

PACT strictly forbids floating-point numbers for financial amounts (all currency is calculated in integer cents). New contributors need explicit examples in the developer guide to avoid common floating-point mistakes.

📍 Where to work

• docs/DEVELOPMENT.md (Section 7: Development Best Practices)
• src/lib/money.ts (Reference existing exported functions: formatCentsToDollars, dollarsToCents, sumCents)

🧭 Implementation guidance

1. Open docs/DEVELOPMENT.md and navigate to Section 7 ("Development Best Practices").
2. Under item 1 ("Integer-Cents for Currency"), add a fenced TypeScript code block showing:
   • Converting dollars string/number to cents using dollarsToCents
   • Formatting integer cents for UI display with formatCentsToDollars
   • Summing multiple transactions safely
3. Ensure the markdown renders cleanly without formatting issues.

✅ Acceptance criteria

• docs/DEVELOPMENT.md contains accurate TypeScript code examples for src/lib/money.ts.
• Code examples use modern TypeScript syntax and import paths.
• Markdown link validation passes with 0 broken links.

🧪 Verification

Start with Section 7 of docs/DEVELOPMENT.md and inspect the exported functions and signatures in src/lib/money.ts: formatCentsToDollars, dollarsToCents, and sumCents. Add copy-pasteable TypeScript examples using the repository’s import conventions, then verify that the Markdown renders cleanly and link validation passes with no broken links.