docs: add agent auth feature proposal

Proposes extending nexus-framework with:
- Agent identity registration and scoped session endpoints
- On-Behalf-Of (OBO) session support with user context stamping
- Custom scope definitions alongside provider OAuth scopes
- Vault backend integration (HashiCorp, AWS SSM, GCP)
- Multi-language SDK support (Python, TypeScript, Go extension)

Includes complete developer journey, accurate code snippets,
implementation priority table, and rationale grounded in
production experience building agent systems.

Author: ekizito96