docs: README's gate list does not match the gate

[OmarAlJarrah]

Problem

README.md's Building section introduces its command block with "These are the same checks the
CI gate runs, and all must pass before a change lands", and three lines in it are not that:

• gofmt -l . — the gate runs gofmt -l $(git ls-files '*.go'). The two differ on any untracked
  or ignored Go file in the tree.
• go test ./... — not a gate step. scripts/check-coverage.sh runs the suite itself, so the
  README lists it twice under different names.
• ./scripts/check-coverage.sh # enforces 100% statement coverage, overall and per package —
  the script counts statements across the whole profile, not per package. A package could be at
  zero and the gate would still pass if the total were covered.

CLAUDE.md carries the same list under the same promise and does not have these three problems,
so the two files disagree about what the gate is.

Acceptance

• The README block matches .github/workflows/gate.yml step for step, in order.
• The per-package claim either becomes true of the script or comes out of the comment.

[OmarAlJarrah]

The Building block is being replaced wholesale in #344, which turns the gate into a Makefile the
workflow drives step by step. The block becomes make gate plus a pointer at the Makefile, so all
three problems above go with it: no gofmt -l ., no standalone go test ./..., and no per-package
coverage claim to be wrong about.

Leaving this open until #344 lands; it closes there rather than here.