chore: add make check and drive the CI lint gate from it

[KristofferRisa]

Goal

Make the CI lint gate reproducible locally, so a contributor can tell whether their branch will pass before they push.

PR #12 shipped a gofmt -s violation through a full review cycle. The reason wasn't carelessness — nothing they could run locally reproduced the gate:

• make fmt ran plain go fmt, which does not apply -s. The target named "fmt" did not satisfy the gate named "Check formatting".
• The gate itself only existed inside test.yml, so the only way to run it was to push and wait.
• Fork PRs from new contributors need manual Actions approval, so pushing and waiting returned nothing either. All 16 runs on feat: implement consumption history command #12 sat in action_required.

CONTRIBUTING.md documented the make fmt trap in prose. That wasn't enough — the fix is to remove the trap, not describe it.

Changes

make check runs every gate the lint job enforces:

check: check-fmt vet mod-verify test-race

The workflow now calls those targets instead of duplicating the commands inline, so the Makefile and CI cannot drift:

- name: Check formatting
  run: make check-fmt
- name: Run go vet
  run: make vet
- name: Run go mod verify
  run: make mod-verify

make fmt now runs gofmt -s -w . — it fixes exactly what check-fmt checks.

.PHONY was missing fmt, lint, tidy and the per-platform build targets. All listed now.

Docs: the pre-PR checklist in CONTRIBUTING.md collapses to a single make check, and the paragraph explaining the make fmt trap is deleted because the trap is gone. Added a note that CI runners are UTC, so timestamp tests must pin their zone — #12 also shipped a test asserting against t.Local(), which passed in CI and failed from UTC+11 east.

The test job is unchanged: it needs the coverage flags and the OS matrix, so it stays inline.

Verification

• make check passes.
• make check-fmt fails as expected on deliberately unformatted input, naming the file and pointing at make fmt; make fmt then clears it. This is the gate that silently passed before.
• TZ=Pacific/Auckland go test ./... passes.

Not in this PR

Enabling Actions for outside contributors is a repo setting, not code — Settings → Actions → General → "Fork pull request workflows from outside collaborators". Until that changes, first-time contributors still get zero CI feedback, which was the larger half of what went wrong on #12.

Tasks

• Add make check running the CI gates
• Fix make fmt to apply -s
• Point the workflow's lint job at the Makefile targets
• Update CONTRIBUTING.md and CLAUDE.md
• Change the fork-PR Actions approval setting (manual, repo settings)

Relates to #12

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