Add a single-command `make check` validation loop to templates/

Generated projects have no aggregate verification target. Validation today is lefthook's pre-commit chain plus separate `make lint`, `vet`, `test`, `generate-check`, `frontend-lint`, `frontend-typecheck` and `frontend-audit` targets.

That is a menu, and agents given a menu reliably run fewer checks than they should. `roadmap.md` already refers to `make check` in two places (the *Agent Guardrails (Claude)* and *Static Analysis* rows) as if it exists; it does not.

**Ask:** one `check` target in `templates/Makefile` running the backend and frontend verification steps in a sensible order, so there is a single command an agent (or a human, or CI) can run to know the tree is good.

**Follow-up:** `templates/AGENTS.md` — introduced by #13 — documents the individual targets until this lands, and should name `make check` once it does.

Surfaced while designing #13. See `docs/superpowers/specs/2026-09-04-selectable-agent-guardrails-design.md` (*Out of scope*).

Start in templates/Makefile by reading the existing lint, vet, test, generate-check, frontend-lint, frontend-typecheck, and frontend-audit targets and their dependencies. Add the aggregate check target in a sensible verification order, then update templates/AGENTS.md to document it. Done means make check runs the backend and frontend checks successfully on a generated project.

Written by the indexing model from the issue text.