perf(ci): cross-compile the Go CLI and build dev images amd64-only

The docker-dev build took ~14 min, dominated by a ~12-min server-image build —
its arm64 half runs under QEMU emulation (native better-sqlite3/sharp).

- Dev images (:dev, :dev-cli) now build linux/amd64 only, skipping arm64 QEMU
  entirely. Releases stay multi-arch. (The primary dev consumer runs amd64.)
- The Go CLI builder now runs on $BUILDPLATFORM and cross-compiles via
  GOOS/GOARCH (CGO off), so the release arm64 CLI builds natively instead of
  under QEMU — arm64 'go build' verified at ~5s vs emulated minutes. Adds
  go build/mod cache mounts for fast incremental rebuilds.

.github/workflows/ci.yml

@@ -125,7 +125,9 @@ jobs:
         with:
           context: .
           push: true
-          platforms: linux/amd64,linux/arm64
+          # Dev images are amd64-only to skip slow arm64 QEMU emulation
+          # (native better-sqlite3/sharp). Releases stay multi-arch.
+          platforms: linux/amd64
           build-args: |
             APP_VERSION=dev
             APP_COMMIT=${{ github.sha }}
@@ -139,7 +141,8 @@ jobs:
           context: .
           target: cli
           push: true
-          platforms: linux/amd64,linux/arm64
+          # Dev images are amd64-only (see server image above). Releases stay multi-arch.
+          platforms: linux/amd64
           build-args: |
             APP_VERSION=dev
             APP_COMMIT=${{ github.sha }}