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.

build-logic/convention/build.gradle.kts

@@ -0,0 +1,67 @@
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
+plugins {
+    `kotlin-dsl`
+}
+
+group = "com.example.architecture.buildlogic"
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_17
+    targetCompatibility = JavaVersion.VERSION_17
+}
+
+tasks.withType<KotlinCompile>().configureEach {
+    compilerOptions {
+        jvmTarget = JvmTarget.JVM_17
+    }
+}
+
+dependencies {
+    // The convention plugins apply these by id, so they only need them at compile time.
+    compileOnly(libs.android.gradlePlugin)
+    compileOnly(libs.kotlin.gradlePlugin)
+    compileOnly(libs.compose.compiler.gradlePlugin)
+}
+
+gradlePlugin {
+    plugins {
+        register("androidApplication") {
+            id = "architecture.android.application"
+            implementationClass = "com.example.architecture.convention.AndroidApplicationConventionPlugin"
+        }
+        register("androidLibrary") {
+            id = "architecture.android.library"
+            implementationClass = "com.example.architecture.convention.AndroidLibraryConventionPlugin"
+        }
+        register("androidFeature") {
+            id = "architecture.android.feature"
+            implementationClass = "com.example.architecture.convention.AndroidFeatureConventionPlugin"
+        }
+        register("androidFeatureViews") {
+            id = "architecture.android.feature.views"
+            implementationClass = "com.example.architecture.convention.AndroidFeatureViewsConventionPlugin"
+        }
+        register("domainModule") {
+            id = "architecture.domain.module"
+            implementationClass = "com.example.architecture.convention.DomainModuleConventionPlugin"
+        }
+        register("compose") {
+            id = "architecture.compose"
+            implementationClass = "com.example.architecture.convention.ComposeConventionPlugin"
+        }
+        register("koin") {
+            id = "architecture.koin"
+            implementationClass = "com.example.architecture.convention.KoinConventionPlugin"
+        }
+        register("ktor") {
+            id = "architecture.ktor"
+            implementationClass = "com.example.architecture.convention.KtorConventionPlugin"
+        }
+        register("kotlinxSerialization") {
+            id = "architecture.kotlinx.serialization"
+            implementationClass = "com.example.architecture.convention.KotlinxSerializationConventionPlugin"
+        }
+    }
+}