ci: optimize test matrix — 9 containers down to 4

- Drop Node 20 (EOL April 2026)
- Reduce macOS to single runner (Node 22) — platform compat check
- Reduce Windows to single runner (Node 22) — slowest CI, smoke-test
- Keep Ubuntu × {22, 24} as primary test surface

Estimated savings: ~60% fewer runner-minutes per CI run
(~500s → ~190s, 9 jobs → 4 jobs)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/test.yml

@@ -21,8 +21,15 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
-        node-version: [20, 22, 24]
+        os: [ubuntu-latest]
+        node-version: [22, 24]
+        include:
+          # Single macOS runner — verifies platform compatibility
+          - os: macos-latest
+            node-version: 22
+          # Single Windows runner — slowest CI runner, smoke-test only
+          - os: windows-latest
+            node-version: 22
 
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2