ci(build): restore triggers (push/PR/dispatch) y conservar concurrency

Author: CoderDeltaLAN

.github/workflows/build.yml

@@ -1,14 +1,12 @@
-name: CI / build
-
 on:
   push:
     branches: [main]
   pull_request:
     branches: [main]
   workflow_dispatch:
-permissions:
-  contents: read
-
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
   build:
     name: build (${{ matrix.python-version }})