Add release workflow, LICENSE, and version stamping

Tag-triggered workflow builds multi-platform binaries, pushes a multi-arch
Docker image to GHCR, bumps and releases the Helm chart, and creates a
GitHub release with all binary artifacts. Adds GPL-3.0 LICENSE and version
variable stamped at build time via ldflags.

Dockerfile

@@ -3,7 +3,8 @@ WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-w -s" -o /terdut ./cmd/terdut
+ARG VERSION=dev
+RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-w -s -X main.version=${VERSION}" -o /terdut ./cmd/terdut
 
 FROM scratch
 COPY --from=builder /terdut /terdut