One `make check` target, and it is non-negotiable

Every project gets a single make check that runs lint, format-check and tests for the whole stack at once, and nothing gets committed until it is green.

The value is not the automation, it is that there is exactly one thing to remember and no partial-check escape hatch to negotiate with. Per-side targets exist for speed while iterating, but the gate before a commit is always the full one. Written into the project instructions as mandatory, an agent will run it unprompted for the rest of the session.