Add make help as the default Makefile target

## Task

`Makefile` has 15+ phony targets and no index. `make` with no args is not useful for newcomers.

## Scope

- `.DEFAULT_GOAL := help`
- `help` prints target + one-line description for existing targets (`test`, `test-browser`, `lint`, `prove-real-voice`, etc.)
- Mention `make help` in CONTRIBUTING and ONBOARDING

## Acceptance criteria

- [ ] `make` and `make help` print the list
- [ ] Existing target recipes unchanged

Start with the Makefile and inspect its existing phony targets, including test, test-browser, lint, and prove-real-voice. Add the default help behavior and document make help in CONTRIBUTING and ONBOARDING, while leaving existing recipes unchanged. Run make and make help to confirm both print the target list.

Written by the indexing model from the issue text.