[OpenSpec] Implement global config.yaml for workflow rules

Currently, custom workflow rules defined in openspec/config.yaml are project-level only. There is no way to set global workflow rules that apply automatically to every new project. Use case: When starting a new project, users have to manually copy their config.yaml rules from a previous project or recreate them from scratch. This creates friction and inconsistency across projects. Suggested improvement: Allow custom rules defined in a global config to be automatically inherited by every new project, similar to how global profile and tool settings work today. This would be particularly useful for teams and individuals who want to enforce consistent workflow discipline across all their projects without manual setup each time. Example rules that would benefit from global support: - Workflow sequence enforcement - Artifact review confirmation requirements - Coding standards and conventions