Rename flutter to flutter-sdk, add curl to runtime, add update-check pipeline

- Rename flutter variant to flutter-sdk for clarity across all configs
  and pipelines
- Add curl to dotnet-runtime apko config (needed to bootstrap .NET
  runtime installation in downstream Dockerfiles)
- Add daily update-check pipeline that monitors Flutter stable channel
  and Wolfi package updates, auto-creates releases for new Flutter
  versions and rebuilds all variants with latest packages

Tested all variants with real workloads:
- dotnet-sdk: dotnet new console + build + run
- dotnet-runtime: multi-stage build, run prebuilt app
- flutter-sdk: flutter create + build web --release

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apko/flutter-sdk.yaml

@@ -0,0 +1,44 @@
+contents:
+  keyring:
+    - https://packages.wolfi.dev/os/wolfi-signing.rsa.pub
+  repositories:
+    - https://packages.wolfi.dev/os
+  packages:
+    # Base runtime
+    - wolfi-baselayout
+    - glibc
+    - glibc-locale-posix
+    - libstdc++
+    - ca-certificates-bundle
+    - tzdata
+    # Build tools
+    - bash
+    - busybox
+    - coreutils
+    - git
+    - curl
+    - wget
+    - unzip
+    - xz
+
+accounts:
+  groups:
+    - groupname: flutter
+      gid: 65532
+  users:
+    - username: flutter
+      uid: 65532
+      gid: 65532
+  run-as: 65532
+
+archs:
+  - x86_64
+  - aarch64
+
+environment:
+  TZ: UTC
+  FLUTTER_HOME: /opt/flutter
+  PATH: /opt/flutter/bin:/opt/flutter/bin/cache/dart-sdk/bin:/usr/bin:/bin:/usr/sbin:/sbin
+
+entrypoint:
+  command: /bin/bash