Change central default validation stage from advisory to enabled

[hdamker]

Problem description

The central config/validation-settings.yaml on tooling/main sets defaults.stage: advisory. Every repository ready for the full validation gate currently needs an explicit per-repo override to enabled.

Possible evolution

Change the default to enabled:

defaults:
  stage: enabled  # was: advisory
  pr_profile: standard
  release_profile: standard

Per-repo stage: advisory and stage: disabled overrides continue to work for opt-out. Existing per-repo enabled entries become redundant but not harmful; they can be cleaned up opportunistically.

Prerequisite: PR #260 (validation-framework → main merge) lands first.

Additional context

Upstream tracker: ReleaseManagement#484.