fix(ci): Fix deploy workflow branch detection for manual triggers

The deploy workflow was failing when manually triggered via workflow_dispatch
because it only checked for github.event.workflow_run.head_branch, which is
empty for manual triggers.

Issue:
- Manual trigger: gh workflow run deploy.yml --ref refactor/abstraction-layer
- BRANCH_NAME was empty ("")
- git checkout "" failed: "empty string is not a valid pathspec"
- Deployment failed on all Pis

Fix:
- Use fallback operator: github.event.workflow_run.head_branch || github.ref_name
- workflow_run trigger: uses head_branch (branch that triggered the tests)
- workflow_dispatch trigger: uses ref_name (branch being run on)

Now works for both:
- Automatic deployment after tests complete
- Manual deployment via workflow_dispatch or gh CLI

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

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

.github/workflows/deploy.yml

@@ -67,7 +67,7 @@ jobs:
         PI_HOSTS: ${{ secrets.PI_HOSTS }}
         PI_REPO_PATH: ${{ secrets.PI_REPO_PATH }}
         PI_USER: ${{ secrets.PI_USER }}
-        BRANCH_NAME: ${{ github.event.workflow_run.head_branch }}
+        BRANCH_NAME: ${{ github.event.workflow_run.head_branch || github.ref_name }}
       run: |
         # Split comma-separated PI_HOSTS into array
         IFS=',' read -ra HOSTS <<< "$PI_HOSTS"