fix: use self-hosted runner for CI test job to avoid billing costs (#67)

The CI test job was running on macos-latest (GitHub-hosted), costing
~$0.08/min. With 65+ PRs in 2 days this burned the monthly budget.
Switch to self-hosted (Mac mini) which the release workflow already
uses successfully with the same toolchain.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   test:
     name: Tests & Quality Checks
-    runs-on: macos-latest
+    runs-on: self-hosted
 
     steps:
       - uses: actions/checkout@v4