Vendor Neutrality and Dependency Governance Baseline

## Goal
Define vendor-neutral boundaries and dependency governance so all merge-critical paths remain open-source executable.

## Tasks
- Define adapter boundaries between core OSS pipeline and optional proprietary integrations.
- Define approved/disallowed license classes for merge-critical dependencies.
- Define dependency drift review cadence and ownership.
- Define acceptance criteria for optional accelerator onboarding.

## Done When
- At least one fully OSS reference pipeline is documented and passing.
- No merge-critical check depends on proprietary tooling.
- Dependency/license checks are active in CI and auditable.

Parent: https://github.com/eclipse-score/score/issues/2827