fix: restrict v3 workflow to only v3-alpha PRs with v3 changes

Add path filter to ensure Build + Test v3 workflow only runs on
PRs targeting v3-alpha branch that actually modify v3/ directory.
This prevents the workflow from running on master branch PRs.

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

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

Author: leaanthony

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

@@ -5,6 +5,8 @@ on:
     types: [opened, synchronize, reopened, ready_for_review]
     branches:
       - v3-alpha
+    paths:
+      - 'v3/**'
   pull_request_review:
     types: [submitted]
     branches: