refactor(di): split module service composition

[gbaudrit]

Summary

Closes #240.

Split the platform, Web Console, Web host, and Workplace host dependency-injection composition into focused, module-owned extension methods while preserving the public AddAgentstration compatibility overload.

Changes

• added a cohesive AgentstrationServiceRegistrationOptions entry point and ten focused platform composition extensions
• extracted Web Console, Web host, and Workplace host registrations from broad roots into owned extensions
• removed the duplicate IRuntimeRunExecutionScope descriptor and made fallback/replacement semantics explicit with TryAdd and Replace
• added descriptor-cardinality, provider-selection, ValidateOnBuild, host lifecycle, and architecture guard tests
• documented the composition ownership map and registration policy
• added CAR-0240 for the ai-defect analysis and prevention record

Validation

• git diff --check
• GitHub Actions CI run 507: formatting, full build, MSTest suite, Source Registry smoke test, PostgreSQL migration/restart and concurrency validation, container build, and Windows host lifecycle — passed
• Dependency Review run 381 — passed
• Documentation run 337 — passed
• CodeQL run 503 — passed

Breaking changes

None. The existing AddAgentstration overload remains available and delegates to the new options-based façade.