feat(ex05): app upgrade guide — GitOps loop reflection and drift demo

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+# Exercise 05 — App Upgrade via GitOps
+
+**Time**: ~15 min (often done as the final step of Exercise 04)
+**Goal**: Reflect on the complete GitOps loop you've built and optionally run another upgrade cycle.
+
+---
+
+## What you built
+
+You now have a fully functioning GitOps platform:
+
+```
+Git repo (source of truth)
+      │
+      │  ArgoCD polls every 3 min (or on Refresh)
+      ▼
+ArgoCD (GitOps engine)
+      │  detects drift between Git and cluster
+      ▼
+Kubernetes cluster
+      │  MetalLB assigns LAN IP to Ingress-Nginx
+      ▼
+Ingress-Nginx (routes by hostname)
+      │
+      ├─► podinfo.192.168.56.200.nip.io  →  podinfo Deployment
+      └─► argocd.192.168.56.200.nip.io   →  ArgoCD UI
+```
+
+And a CI pipeline that closes the loop:
+
+```
+Tekton PipelineRun
+      │
+      ├─ validate manifests
+      ├─ bump image tag in deployment.yaml
+      └─ git push
+            │
+            ▼
+      ArgoCD detects commit → syncs → rolling update
+```
+
+---
+
+## Reflect: What makes this "GitOps"?
+
+1. **Git is the source of truth** — the cluster state is always derived from this repo
+2. **No manual kubectl apply** — all cluster changes go through Git commits
+3. **Drift detection** — if someone manually changes something in the cluster, ArgoCD reverts it
+4. **Audit trail** — every cluster change has a corresponding Git commit
+5. **Rollback = git revert** — no special tooling needed
+
+---
+
+## Optional: Try a manual upgrade
+
+If the pipeline already bumped podinfo to `6.7.0`, try a manual downgrade to see
+the loop in reverse:
+
+```bash
+# Edit the image tag back to 6.6.2
+vim manifests/apps/podinfo/deployment.yaml
+# Change: ghcr.io/stefanprodan/podinfo:6.7.0
+# To:     ghcr.io/stefanprodan/podinfo:6.6.2
+
+git add manifests/apps/podinfo/deployment.yaml
+git commit -m "chore: downgrade podinfo to 6.6.2 for demo"
+git push
+```
+
+Watch ArgoCD sync in the UI, then verify:
+
+```bash
+curl http://podinfo.192.168.56.200.nip.io | jq .version
+# "6.6.2"
+```
+
+Now upgrade again via the pipeline:
+
+```bash
+kubectl delete pipelinerun bump-podinfo-to-670 -n tekton-pipelines
+kubectl apply -f manifests/ci/pipeline/pipelinerun.yaml
+```
+
+---
+
+## Optional: Test drift detection
+
+ArgoCD's `selfHeal: true` means it will automatically revert manual cluster changes.
+
+Try bypassing GitOps:
+
+```bash
+# Change the image tag directly in the cluster (not via Git)
+kubectl set image deployment/podinfo podinfo=ghcr.io/stefanprodan/podinfo:6.5.0 -n podinfo
+```
+
+Watch the ArgoCD UI — within seconds you'll see the `podinfo` app go **OutOfSync**,
+then ArgoCD reverts it back to whatever tag is in Git. The cluster drifted; GitOps corrected it.
+
+---
+
+## Summary
+
+| Component | Purpose | How deployed |
+|-----------|---------|-------------|
+| k3s | Kubernetes | Vagrantfile |
+| ArgoCD | GitOps engine | bootstrap.sh → self-manages |
+| MetalLB | LoadBalancer IPs | ArgoCD |
+| Ingress-Nginx | HTTP routing | ArgoCD |
+| podinfo | Demo app | ArgoCD |
+| Tekton | CI pipeline | ArgoCD |
+
+---
+
+## What's next
+
+If you have time, try **Exercise 06 (Bonus)**: deploy Prometheus + Grafana and
+observe your cluster and podinfo metrics in a live dashboard.
+
+Otherwise, join the **final presentation** for a discussion on:
+- Why GitOps in production
+- What comes next: Vault, ApplicationSets, Argo Rollouts