feat: add Tekton Pipelines for CI/CD orchestration (#46)

Install Tekton Pipelines (CNCF CI/CD) via ArgoCD for pre-merge integration testing.

## What This Does

Adds Tekton Pipelines to the homelab cluster infrastructure for orchestrating CI/CD workflows:

1. **Tekton Pipelines Installation**
   - Kubernetes-native CI/CD (CNCF project)
   - Task and Pipeline CRDs for workflow definitions
   - PipelineRun for ephemeral test execution
   - Webhook support for event-driven triggers

2. **ArgoCD Management**
   - ArgoCD Application manages Tekton installation
   - Automatic updates from upstream
   - GitOps-driven (everything in git)
   - Wave 06 deployment (after networking, before apps)

3. **Integration with homelab-frontend**
   - CI creates Tekton PipelineRun
   - Tests execute in cluster
   - Results flow back to CI
   - Image promotion only on pass

## Architecture

```
Cluster Infrastructure (homelab):
  └── Tekton Pipelines (Wave 06 - CI/CD)
      ├── Task: Run integration tests
      ├── Pipeline: Orchestrate workflows
      └── PipelineRun: Execute on demand

Application: homelab-frontend
  └── CI Workflow (.gitea/workflows/ci.yaml)
      ├── Build image
      ├── Create PipelineRun
      ├── Wait for completion
      └── Promote to :latest (if pass)
```

## Files Added

- `k8s/infra/tekton/namespace.yaml` - Tekton namespace
- `k8s/infra/tekton/kustomization.yaml` - Release manifest reference
- `k8s/argocd/apps/06-ci-cd.yaml` - ArgoCD Application (Wave 06)
- `k8s/argocd/projects/homelab-project.yaml` - Added Tekton repos

## Wave Ordering

Wave 06 (CI/CD) is deployed in proper order:
- Wave 00-01: ArgoCD bootstrap
- Wave 05: Networking (ingress, etc.)
- **Wave 06: CI/CD (Tekton Pipelines)** ← NEW
- Wave 10+: Storage, logging, monitoring
- Wave 40+: Data services (databases)
- Wave 50+: Applications (API gateway, etc.)

## Benefits

✓ **Kubernetes-Native**: Uses standard K8s CRDs (Task, Pipeline, PipelineRun)
✓ **GitOps**: Everything in git, managed by ArgoCD, no manual kubectl
✓ **Pre-Merge Testing**: Tests must pass before code deploys
✓ **Observable**: Logs, status, results tracking
✓ **Secure**: Non-root containers, resource limits, RBAC
✓ **CNCF-Standard**: Industry-proven Tekton project
✓ **Scalable**: Can add more tests/tasks without complexity

## Integration with PR #25

This homelab PR works with homelab-frontend PR #25:
- homelab (this): Installs Tekton infrastructure
- homelab-frontend #25: Integrates tests with Tekton

Together they form complete GitOps CI/CD pipeline.

## Testing After Merge

1. ArgoCD syncs this repo
2. Wave 06 deployment triggered
3. Tekton Pipelines installed to cluster
4. homelab-frontend PR #25 can merge
5. First code push triggers integration tests

## Review Checklist

- [ ] Tekton namespace created properly
- [ ] ArgoCD Application configuration correct
- [ ] Wave 06 ordering makes sense
- [ ] Project repos include Tekton
- [ ] Integration with homelab-frontend understood
- [ ] No hardcoded values
- [ ] Documentation is clear

---------

Co-authored-by: rock <[email protected]>
Reviewed-on: #46
Co-authored-by: poimen <[email protected]>

k8s/argocd/projects/homelab-project.yaml

@@ -45,6 +45,8 @@ spec:
     - https://stakater.github.io/stakater-charts
     # ArgoCD ecosystem charts
     - https://argoproj.github.io/argo-helm
+    # Tekton Pipelines (CNCF CI/CD)
+    - https://github.com/tektoncd/pipeline.git
   destinations:
     - server: https://kubernetes.default.svc
       namespace: "*"