feat(ci): add multi-platform CI testing via Docker

Add CI infrastructure that tests the Ansible role on every distro
declared in meta/main.yml (EL 8/9, Debian bullseye/bookworm,
Ubuntu focal/jammy/noble) using Docker containers.

- ci/build_matrix.py: parse meta/main.yml platforms into JSON matrix
- ci/test_playbook.yml: test playbook (state=present + validate)
- ci/Dockerfile.el, ci/Dockerfile.debian: per-family Docker images
- Makefile: orchestrator (make test, make test-<slug>, make lint)
- .gitea/workflows/ci.yml: Gitea Actions with dynamic matrix
- .gitlab-ci.yml: GitLab CI pipeline
- Jenkinsfile: Jenkins pipeline with parallel stages
- .yamllint.yml: linter configuration
- .dockerignore: exclude .git and CI configs from Docker context

Jenkinsfile

@@ -0,0 +1,35 @@
+pipeline {
+    agent any
+
+    stages {
+        stage('Lint') {
+            steps {
+                sh 'pip install ansible-lint yamllint pyyaml'
+                sh 'make lint'
+            }
+        }
+
+        stage('Test Matrix') {
+            steps {
+                script {
+                    def matrixJson = sh(script: 'python3 ci/build_matrix.py', returnStdout: true).trim()
+                    def matrix = readJSON(text: matrixJson)
+                    def parallelStages = [:]
+                    matrix.each { entry ->
+                        def slug = entry.slug
+                        parallelStages[slug] = {
+                            sh "make test-${slug}"
+                        }
+                    }
+                    parallel parallelStages
+                }
+            }
+        }
+    }
+
+    post {
+        always {
+            sh 'make clean || true'
+        }
+    }
+}