chore: scaffold multi-module project, version catalog, and build-logic

Foundation milestone (REDI-78, REDI-79):

- Multi-module skeleton: :app, :core:{domain,data,presentation,design-system},
  :feature:characters:{domain,data,presentation,presentation-compose,presentation-views},
  :feature:about:presentation, plus the :build-logic composite build.
- gradle/libs.versions.toml as the single source of truth ([versions]/[libraries]/
  [bundles]/[plugins]); no inline versions in any build file.
- Convention plugins: architecture.android.{application,library,feature,feature.views},
  domain.module, compose, koin, ktor, kotlinx.serialization.
- Pure-Kotlin domain modules; presentation-compose uses android.feature;
  presentation-views uses android.feature.views (ViewBinding on, Compose off);
  the UI-agnostic :presentation has neither Compose nor Views deps.
- Toolchain: AGP 9.0.1, Kotlin 2.3.20, Gradle 9.1.0, compileSdk 36, minSdk 24, Java 17.
- Minimal MainActivity placeholder; CI (assembleDebug) via GitHub Actions.

Verified: ./gradlew projects lists the full tree and ./gradlew assemble is green.

.gitignore

@@ -0,0 +1,27 @@
+*.iml
+.DS_Store
+
+# Gradle
+**/.gradle/
+**/build/
+/captures
+
+# Kotlin
+.kotlin/
+
+# Native
+.externalNativeBuild
+.cxx
+
+# Local config / secrets
+local.properties
+
+# IDE
+/.idea/caches
+/.idea/libraries
+/.idea/modules.xml
+/.idea/workspace.xml
+/.idea/navEditor.xml
+/.idea/assetWizardSettings.xml
+/.idea/deploymentTargetSelector.xml
+/.idea/deploymentTargetDropDown.xml