Remove standalone Tilt environment

Drop the standalone Tilt and Kubernetes development environment and
keep only the configuration needed to run this project through Enduro's
dev env. Flatten the Kubernetes manifests into a single kustomize root,
update the README to describe both child workflows, and replace the
Tilt CI workflow with manifest and Tiltfile validation.

Author: jraddaoui

.github/workflows/tilt-ci.yml

@@ -0,0 +1,20 @@
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+name: Validate Tilt
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+      - name: Install kubectl and Tilt
+        uses: yokawasa/action-setup-kube-tools@v0.13.4
+        with:
+          setup-tools: |
+            kubectl
+            tilt
+      - name: Validate Tiltfile
+        run: make validate-tilt

.github/workflows/validate-tilt.yml

@@ -158,3 +158,6 @@ tool-%:
 tools: # @HELP Install all tools managed by bine.
 tools:
 	go tool bine sync
+
+validate-tilt: # @HELP Validate the Tiltfile and Kubernetes manifests.
+	tilt alpha tiltfile-result > /dev/null