fix(model): handle YAML map forms that caused unmarshall errors

Variables with value/description/options sub-keys, default.image in map
form, default.before_script / default.after_script as block scalars, and
rules.changes / rules.exists in {paths, compare_to} map form all caused
"yaml: cannot unmarshal !!map into string" because the struct fields were
typed too narrowly.

Changed types in model.Pipeline, model.DefaultConfig, and model.Rule to
accept any to match GitLab CI spec flexibility (13.7+ variable declarations,
15.3+ rules.changes map form, image map form in default block).

Adds testdata/script_multiline.yml covering all these patterns.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

testdata/script_multiline.yml

@@ -0,0 +1,87 @@
+---
+# Exercises multi-line script patterns and extended variable declarations.
+# Ref: https://docs.gitlab.com/ci/yaml/script/#split-long-commands
+# Ref: https://docs.gitlab.com/ee/ci/yaml/#variablesdescription
+# All patterns here must parse cleanly (exit 0).
+
+stages:
+  - build
+  - test
+  - deploy
+
+# Pipeline-level variables: plain strings and extended {value, description} map form.
+variables:
+  PLAIN_VAR: "hello"
+  DEPLOY_ENV:
+    value: "staging"
+    description: "The deployment target. Set to staging or production."
+  RETRIES:
+    value: "3"
+    description: "Number of retry attempts."
+    options:
+      - "1"
+      - "3"
+      - "5"
+
+default:
+  # image in map form (name + pull_policy)
+  image:
+    name: alpine:latest
+    pull_policy: if-not-present
+  # before_script as a block scalar (not a list)
+  before_script:
+    - apk add --no-cache curl git
+
+build-literal-block:
+  stage: build
+  # script items using literal block scalar (|)
+  script:
+    - |
+      if [[ "$DEPLOY_ENV" == "production" ]]; then
+        echo "Production build"
+      else
+        echo "Non-production build"
+      fi
+    - echo "Build step done"
+
+build-folded-block:
+  stage: build
+  # script items using folded block scalar (>)
+  script:
+    - >
+      apt-get update -qq &&
+      apt-get install -y curl wget
+    - echo "Packages installed"
+  before_script:
+    - |
+      echo "Job-level before_script"
+      echo "Using literal block scalar"
+
+test-job:
+  stage: test
+  script:
+    - echo "Running tests"
+    - |
+      set -e
+      go test ./...
+      echo "Tests passed"
+  # Job-level variable with extended form
+  variables:
+    TEST_FLAG:
+      value: "true"
+      description: "Enable verbose test output"
+  # rules.changes in map form (GitLab 15.3+)
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+      changes:
+        paths:
+          - "**/*.go"
+        compare_to: "main"
+      when: on_success
+    - when: on_success
+
+deploy-job:
+  stage: deploy
+  script:
+    - echo "Deploying to $DEPLOY_ENV"
+  when: manual