isolated projects: migrate wireToRootAggregator away from rootProject access

[kirich1409]

Per docs/specs/2026-05-16-gradle-plugin-cc-support.md AC-8 deferral.

Currently FeaturedPlugin.kt:157 calls target.rootProject which works under Configuration Cache but breaks isolated projects.

Migration path: convert to settings plugin or have consumers register the aggregator once in root build.gradle.kts.

Spec exclude: out of scope for v1.0.0-Beta.

[kirich1409]

Fixed in #286 (merged to develop). Plugin no longer accesses rootProject; scanAllLocalFlags removed in favor of built-in ./gradlew resolveFeatureFlags. PI-safety proven by ProjectIsolationIntegrationTest.