go.mod's 'go 1.25.0' directive (auto-set by go mod tidy because
testcontainers-go v0.42.0 requires it) made the runner's bundled
golangci-lint (built on Go 1.24) refuse to load the config.
Pin to v2.12.2, the latest at time of writing, which is built on
Go 1.25.
Also: previous gofmt commit had subject 'style: gofmt -w' which our
custom commitlint regex rejects (style isn't in the type allowlist).
Squashed into this commit so the PR's commits all pass commitlint.
Refs: M4.1
Minimal Go service: /healthz + /v1/tenants/by-slug/:slug + /v1/tenants/:id with an in-memory store seeded with the acme tenant. Stdlib-only; pgx + JWT validation land in M4.1 follow-up.
sharang