docs: expand CONTRIBUTING.md with code quality standards

[sonukapoor]

Closes #445

Expands the coding standards section with the practices we enforce during review but hadn't written down yet. Gives contributors a clear reference before they open a PR.

Added sections:

• Named constants — magic strings and numbers must be extracted to src/constants.ts
• DRY — repeated logic belongs in a shared utility, not copied across files
• File size and single responsibility — keep files focused; src/index.ts stays a thin orchestration layer; new output formats get their own module
• Braces on conditionals — all if/else/for/while blocks require braces
• Formatting — references npm run format and the forthcoming Prettier setup (chore: add Prettier for consistent code formatting #446)