Fix GitHub Actions workflow: update branch to main and GHC to 9.12.2

- Change workflow trigger from 'master' to 'main' branch
- Update GHC version from 8.10.3 to 9.12.2 (matching modernization work)
- Update setup action from actions/setup-haskell@v1 to haskell-actions/setup@v2
- Set cabal-version to 'latest' instead of '3.2'

Author: claude

.github/workflows/haskell.yml

@@ -2,9 +2,9 @@ name: Haskell CI
 
 on:
   push:
-    branches: [ "master" ]
+    branches: [ "main" ]
   pull_request:
-    branches: [ "master" ]
+    branches: [ "main" ]
 
 permissions:
   contents: read
@@ -16,10 +16,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
-    - uses: actions/setup-haskell@v1
+    - uses: haskell-actions/setup@v2
       with:
-        ghc-version: '8.10.3'
-        cabal-version: '3.2'
+        ghc-version: '9.12.2'
+        cabal-version: 'latest'
 
     - name: Cache
       uses: actions/cache@v3