feat: split Dockerfile into dedicated lint stage for faster CI (#32)

## Summary

Split the Dockerfile into a dedicated lint stage using the prebuilt `golangci/golangci-lint:v2.1.6` image, so lint failures are reported faster without needing to download/compile golangci-lint first.

## Changes

- **New lint stage** (`AS lint`): Uses the prebuilt `golangci/golangci-lint` image (pinned by sha256). Runs `make fmt-check` and `make lint`.
- **Build stage** (`AS builder`): Runs `make test` + compilation. No longer installs golangci-lint via `go install`.
- **`COPY --from=lint`**: Forces BuildKit to execute the lint stage before proceeding with the build.
- **Runtime stage**: Unchanged.

All base images remain pinned by sha256 hash.

closes #27

<!-- session: agent:sdlc-manager:subagent:76cebdf6-86f0-4383-93e3-ff3e10fbc7a6 -->

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #32
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

Dockerfile

@@ -1,24 +1,35 @@
+# Lint stage — fast feedback on formatting and lint issues
+# golangci/golangci-lint:v2.1.6, 2026-03-02
+FROM golangci/golangci-lint@sha256:568ee1c1c53493575fa9494e280e579ac9ca865787bafe4df3023ae59ecf299b AS lint
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN make fmt-check
+RUN make lint
+
+# Build stage
 # golang:1.24-alpine, 2026-02-26
 FROM golang@sha256:8bee1901f1e530bfb4a7850aa7a479d17ae3a18beb6e09064ed54cfd245b7191 AS builder
 WORKDIR /src
 RUN apk add --no-cache git build-base make
 
-# golangci-lint v2.1.6 (eabc2638a66d), 2026-02-26
-RUN CGO_ENABLED=0 go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@eabc2638a66daf5bb6c6fb052a32fa3ef7b6600d
+# Force BuildKit to run the lint stage before proceeding
+COPY --from=lint /src/go.sum /dev/null
 
 COPY go.mod go.sum ./
 RUN go mod download
 
 COPY . .
 
-# Run all checks — build fails if branch is not green
-RUN make check
+RUN make test
 
 # Build static binaries (no cgo needed at runtime — modernc.org/sqlite is pure Go)
 ARG VERSION=dev
 RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w -X main.Version=${VERSION}" -o /chatd ./cmd/chatd/
 RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /chat-cli ./cmd/chat-cli/
 
+# Runtime stage
 # alpine:3.21, 2026-02-26
 FROM alpine@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
 RUN apk add --no-cache ca-certificates \