ci: wire dgb/slice4-skeleton into PR trigger so stacked PR #113 runs the matrix

Add dgb/slice4-skeleton to pull_request.branches in build.yml and
coin-matrix.yml. The filter is read from the base branch copy, so PRs
targeting this integration branch (#113 dgb/share-layer-port) otherwise
never trigger. Additive only; per-coin source-presence guard unchanged.

Author: frstrtr

.github/workflows/build.yml

@@ -5,7 +5,7 @@ on:
     branches: [master, btc-embedded, dash-spv-embedded, windows-build, service-update]
     tags: ['v*']
   pull_request:
-    branches: [master, btc-embedded, dash-spv-embedded]
+    branches: [master, btc-embedded, dash-spv-embedded, dgb/slice4-skeleton]
 
 jobs:
   # ════════════════════════════════════════════════════════════════════════════

.github/workflows/coin-matrix.yml

@@ -15,7 +15,7 @@ on:
   push:
     branches: [master, btc-embedded, dash-spv-embedded, windows-build, service-update]
   pull_request:
-    branches: [master, btc-embedded, dash-spv-embedded]
+    branches: [master, btc-embedded, dash-spv-embedded, dgb/slice4-skeleton]
 
 jobs:
   coin: