docs: expand CONTRIBUTING.md with code quality standards

[sonukapoor]

The CONTRIBUTING.md coding standards section should be expanded to document the practices we enforce during review but haven't written down yet.

Additions needed:

• Named constants: magic strings and numbers must be extracted to named constants in src/constants.ts rather than inlined
• DRY: repeated string literals, logic blocks, and utility functions must be extracted to shared modules — not duplicated across files
• Single responsibility: each file should have one clear purpose; if it can't be described in one sentence, extract
• File size: keep files focused and small; extract shared logic into utilities rather than continuing to add to a large file
• New output formats: follow the html-reporter.ts pattern — dedicated file per output format
• Keep src/index.ts lean: display and formatting logic belongs in utility modules, not inlined in the entry point