fix: add explicit permissions to workflows

Resolves CodeQL alert: workflows without explicit permissions run
with broad defaults. Restrict to contents:read.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj

.github/workflows/build.yml

@@ -5,6 +5,9 @@ on:
     branches:
       - main
 
+permissions:
+  contents: read
+
 jobs:
   build:
     runs-on: ubuntu-latest

.github/workflows/lighthouse.yml

@@ -5,6 +5,9 @@ on:
     branches:
       - main
 
+permissions:
+  contents: read
+
 jobs:
   lighthouse:
     runs-on: ubuntu-latest