feat(infra): add local pull-through mirrors for NuGet, npm, Docker Hub

docker-compose.mirror.yml:
  - BaGet  (port 5101) → proxies nuget.org
  - Verdaccio (port 4873) → proxies npmjs.com
  - registry:2 (port 5100) → proxies Docker Hub

nuget.mirror.config: points dotnet restore at http://mirror:5101
mirrors/verdaccio/config.yaml: open reads, upstream npmjs fallback

CI workflow:
  - All container jobs: --add-host=mirror:host-gateway
  - dotnet restore --configfile nuget.mirror.config
  - npm install --registry http://mirror:4873

First run: packages fetched from upstream through the VPS.
All subsequent runs: served from local disk, no CDN needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nuget.mirror.config

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- NuGet config for CI: routes all restores through the local BaGet mirror.
+     Usage: dotnet restore --configfile nuget.mirror.config
+     BaGet fetches from nuget.org on first request, then caches locally.
+     DO NOT use this file for local development (mirror must be running). -->
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="local-mirror" value="http://mirror:5101/v3/index.json" protocolVersion="3" />
+  </packageSources>
+  <config>
+    <add key="http_retry_count" value="8" />
+    <add key="http_retry_delay_milliseconds" value="1000" />
+  </config>
+</configuration>