docs: add AGENTS.md contributor and agent guide

.github/workflows/build-pr.yml

@@ -99,6 +99,13 @@ jobs:
         run: |
           git diff --exit-code docs/api-reference.md || (echo "API reference is out of date. Run 'make docs'" && exit 1)
 
+      - name: Regenerate AGENTS.md generated blocks
+        run: make agents
+
+      - name: Check for stale AGENTS.md
+        run: |
+          git diff --exit-code AGENTS.md || (echo "AGENTS.md is out of date. Run 'make agents'" && exit 1)
+
   test:
     name: Unit Tests
     runs-on: ubuntu-latest
@@ -191,6 +198,11 @@ jobs:
 
       - name: Set up Helm
         uses: azure/setup-helm@v5
+        with:
+          # Pin to Helm 3.x: setup-helm's "latest" now installs Helm 4, whose
+          # multi-document template output differs (trailing newline), which
+          # breaks the nebari-app golden tests. The goldens are Helm 3-rendered.
+          version: v3.19.0
 
       - name: Install foundational services
         run: dev/scripts/services/install.sh
@@ -247,6 +259,11 @@ jobs:
 
       - name: Set up Helm
         uses: azure/setup-helm@v5
+        with:
+          # Pin to Helm 3.x: setup-helm's "latest" now installs Helm 4, whose
+          # multi-document template output differs (trailing newline), which
+          # breaks the nebari-app golden tests. The goldens are Helm 3-rendered.
+          version: v3.19.0
 
       - name: Install kubebuilder
         run: |