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>

Taskfile.yml

@@ -45,6 +45,8 @@ tasks:
         ignore_error: false
       - cmd: ./{{.BINARY}} check testdata/includes_component.yml
         ignore_error: false
+      - cmd: ./{{.BINARY}} check testdata/script_multiline.yml
+        ignore_error: false
       - cmd: ./{{.BINARY}} check testdata/context_rules.yml
         ignore_error: false
       - cmd: ./{{.BINARY}} check --branch main testdata/context_rules.yml