ci: migrate to flat runner labels [self-hosted, sylphx, {platform}, {size}]

New canonical format separates platform and size as independent flat labels.
Backward-compatible: old compound labels still work during transition.

Old: sylphx-linux-xlarge, sylphx-standard, [self-hosted, macOS, X64], etc.
New: [self-hosted, sylphx, linux, xlarge], [self-hosted, sylphx, macos, standard], etc.

Author: shtse8

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   ci:
-    runs-on: sylphx-linux-standard
+    runs-on: [self-hosted, sylphx, linux, standard]
     steps:
       - uses: actions/checkout@v4