Add dd0c/drift notifications, infra, CI: Slack Block Kit, Dockerfiles, Gitea Actions

- Notification service: Slack Block Kit (remediate/accept buttons), webhook delivery, rate limit handling
- Dispatcher with severity-based channel filtering
- Agent Dockerfile: multi-stage Go build, static binary
- SaaS Dockerfile: multi-stage Node build
- Fly.io config: scale-to-zero, shared-cpu
- Gitea Actions: Go test+vet, Node typecheck+test, cross-compile agent (linux/darwin/windows)

products/02-iac-drift-detection/agent/Dockerfile

@@ -0,0 +1,13 @@
+# --- Build stage ---
+FROM golang:1.22-alpine AS builder
+WORKDIR /app
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -o /drift ./cmd/drift
+
+# --- Runtime stage ---
+FROM alpine:3.19
+RUN apk add --no-cache ca-certificates
+COPY --from=builder /drift /usr/local/bin/drift
+ENTRYPOINT ["drift"]