fix: trigger CI workflows on master, not just main (#39)

master is where real development happens and gets pushed to directly
(main is the auto-synced mirror) - branch protection was just added to
master requiring these exact check contexts, but the workflows only
fired for PRs/pushes targeting main, so the required checks could never
actually run for a master PR, permanently blocking every future merge.

.github/workflows/code-quality.yml

@@ -2,9 +2,9 @@ name: Code Quality
 
 on:
   push:
-    branches: [main]
+    branches: [main, master]
   pull_request:
-    branches: [main]
+    branches: [main, master]
 
 jobs:
   python-lint:

.github/workflows/security.yml

@@ -2,9 +2,9 @@ name: Security
 
 on:
   push:
-    branches: [main]
+    branches: [main, master]
   pull_request:
-    branches: [main]
+    branches: [main, master]
   schedule:
     - cron: '0 6 * * 1'