feat: add linter configs, Dockerfile, CI/CD pipeline

- .golangci.yml — 50+ linters enabled
- .yamllint — yaml lint rules
- cicd.yaml — CI/CD with lint-go (golangci, staticcheck,
  gosec, revive), lint-yaml, build+test, publish
- Dockerfile — multi-stage scratch build
- go vet passes clean

Dockerfile

@@ -0,0 +1,12 @@
+FROM golang:1.23-alpine AS builder
+RUN apk add --no-cache git ca-certificates
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -ldflags="-s -w" -o /gosleep-timer .
+
+FROM scratch
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
+COPY --from=builder /gosleep-timer /gosleep-timer
+ENTRYPOINT ["/gosleep-timer"]