fix(ci): Add explicit permissions to workflow

Add explicit permissions block to ubuntu-build.yml workflow following
GitHub Actions security best practices. This applies the principle of
least privilege by restricting the workflow to only the permissions it
needs (contents: read).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: 2 people

.github/workflows/ubuntu-build.yml

@@ -12,6 +12,9 @@ on:
   pull_request:
     branches: [ "main" ]
 
+permissions:
+  contents: read
+
 jobs:
 
   build: