ci: add pull_request triggers to CI workflows

Enable PR-triggered CI so required status checks can gate merges.
Spinifex runs on all PRs (no path filter) to avoid blocking non-Go PRs.
UI workflow keeps path filters as it's not a required check.

.github/workflows/spinifex.yml

@@ -11,6 +11,10 @@ on:
       - go.sum
       - Makefile
       - .github/workflows/spinifex.yml
+  pull_request:
+    branches:
+      - main
+      - dev
   workflow_dispatch:
 
 jobs:

.github/workflows/ui.yml

@@ -8,6 +8,13 @@ on:
     paths:
       - spinifex/services/spinifexui/frontend/**
       - .github/workflows/ui.yml
+  pull_request:
+    branches:
+      - main
+      - dev
+    paths:
+      - spinifex/services/spinifexui/frontend/**
+      - .github/workflows/ui.yml
   workflow_dispatch:
 
 jobs: