refactor: lean down src/index.ts after extraction PRs land

[sonukapoor]

Once the individual extraction PRs land (time utilities, package.json helpers, fix execution, CLI validation, formatters), src/index.ts should be a thin orchestration layer of roughly 150–200 lines.

This issue tracks the final cleanup pass after those PRs merge:

• Confirm main() only calls functions, contains no string literals for user-facing output, and has no inline logic that belongs elsewhere
• Remove any leftover helper code that escaped earlier extractions
• Verify the module-level structure (arg parsing, command dispatch, main() call) reads cleanly
• Run npm test && npm run build

Depends on: the time utilities, package.json helpers, fix execution, CLI validation, and formatter extraction issues.

Do not start this until those PRs are merged.