Consolidate collection manifests to single source of truth

Summary

Consolidate collection infrastructure from multiple *.collection.yml and *.collection.md files into a single core-manifest.yml that serves as the source of truth for all collection membership, maturity, and metadata.

Motivation

• Multiple collection files created duplication and drift risk
• Collection membership and maturity tracking was scattered
• Plugin generation required scanning multiple sources

Changes

• Introduce collections/core-manifest.yml as canonical registry
• Project *.collection.yml / *.collection.md in-memory from manifest (no longer committed)
• Add maturity-dependency validation to prevent experimental artifacts from being bundled in stable collections
• Update plugin generation tooling to consume single manifest
• Add plugin parity check and test fixtures

Acceptance Criteria

• core-manifest.yml contains all artifact paths, kinds, collection membership, and maturity
• npm run plugin:generate produces correct outputs from manifest
• npm run lint:collections-metadata validates manifest structure
• Documentation updated to reflect new architecture

Start with the proposed collections/core-manifest.yml and inspect the existing collection files and plugin generation tooling. Run npm run plugin:generate and npm run lint:collections-metadata to understand current outputs and validation. Done means the manifest contains all required metadata, generation and parity checks pass, and the architecture documentation is updated.