fix(ci): add fetch-depth:0 so origin/main available for PR diff

Shallow checkout (depth=1) means origin/main isn't fetched and HEAD~1
doesn't exist — git diff exits 128, failing the detect step.

With fetch-depth:0 all history is present so origin/main...HEAD works.
Also switch fallback to echo "" so it degrades to @smoke not a hard fail.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: lane711

.github/workflows/pr-tests.yml

@@ -37,6 +37,7 @@ jobs:
         with:
           # For pull_request_target, we need to checkout the PR head
           ref: ${{ github.event_name == 'pull_request_target' && github.event.pull_request.head.sha || github.sha }}
+          fetch-depth: 0
 
       - name: Setup Node.js
         uses: actions/setup-node@v4
@@ -194,7 +195,7 @@ jobs:
         run: |
           # Get changed files vs base branch (PR diff) or last commit (push)
           if [ "${{ github.event_name }}" = "pull_request_target" ]; then
-            CHANGED=$(git diff --name-only origin/${{ github.base_ref }}...${{ github.event.pull_request.head.sha }} 2>/dev/null || git diff --name-only HEAD~1)
+            CHANGED=$(git diff --name-only origin/${{ github.base_ref }}...HEAD 2>/dev/null || echo "")
           else
             CHANGED=$(git diff --name-only HEAD~1 2>/dev/null || echo "")
           fi