docs: propose Workspace repository onboarding

[pcvantol]

Summary

• add the canonical proposed Workspace design for repository onboarding and disposable qualification
• define Workspace/Forge/CENTRAL/Project-Agent authority boundaries and the portable repository declaration constraints
• link the proposal into architecture, roadmap, backlog, and validation

Validation

• git diff --check
• bash scripts/validate.sh

TDE

• Not applicable: Workspace has no implementation or TDE evidence profile; this is a documentation-only foundation change.