feat(release): drop darwin_amd64 (Intel Macs) (#15)

Intel Macs are no longer receiving macOS updates. Drop from release
matrix and re-add to goreleaser ignore list. install.sh now prints
a clear message pointing Intel users to build-from-source. README
manual-install section no longer advertises a darwin_amd64 URL.

.github/workflows/release.yaml

@@ -22,7 +22,6 @@ jobs:
       rolling-release-tag: "v1"
       platforms: >-
         [
-          {"os":"macos-13","goos":"darwin","goarch":"amd64","platform":"darwin_amd64"},
           {"os":"macos-latest","goos":"darwin","goarch":"arm64","platform":"darwin_arm64"},
           {"os":"ubuntu-latest","goos":"linux","goarch":"amd64","platform":"linux_amd64"},
           {"os":"ubuntu-24.04-arm","goos":"linux","goarch":"arm64","platform":"linux_arm64"},