Prevent v3 workflow from running on PRs against master

Add explicit base_ref checks to ensure the v3 build and test workflow
only runs for PRs targeting the v3-alpha branch.

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

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

.github/workflows/build-and-test-v3.yml

@@ -16,6 +16,7 @@ jobs:
   check_approval:
     name: Check PR Approval
     runs-on: ubuntu-latest
+    if: github.base_ref == 'v3-alpha'
     outputs:
       approved: ${{ steps.check.outputs.approved }}
     steps:
@@ -32,6 +33,7 @@ jobs:
     name: Run JS Tests
     needs: check_approval
     runs-on: ubuntu-latest
+    if: github.base_ref == 'v3-alpha'
     strategy:
       matrix:
         node-version: [20.x]
@@ -90,6 +92,7 @@ jobs:
     name: Run Go Tests v3
     needs: [check_approval, test_js]
     runs-on: ${{ matrix.os }}
+    if: github.base_ref == 'v3-alpha'
     strategy:
       fail-fast: false
       matrix:
@@ -174,6 +177,7 @@ jobs:
     name: Test Templates
     needs: test_go
     runs-on: ${{ matrix.os }}
+    if: github.base_ref == 'v3-alpha'
     strategy:
       fail-fast: false
       matrix: