fix(ci): build non gira su PR, branches filter su pull_request

- build: if: github.event_name != 'pull_request' — evita publish da PR (tokn
  read-only su fork, semantica sbagliata su PR interne)
- pull_request trigger: branches: [main] per coerenza con push trigger

Author: Gabrymi93

.github/workflows/build-context.yml

@@ -13,6 +13,7 @@ on:
       - 'tests/**'
       - 'pyproject.toml'
   pull_request:
+    branches: [main]
     paths:
       - 'src/**'
       - 'tests/**'
@@ -42,6 +43,7 @@ jobs:
 
   build:
     needs: test
+    if: github.event_name != 'pull_request'
     runs-on: ubuntu-latest
     permissions:
       contents: write