feat: exchange managed tokens after register

[hardy-dev-infinilabs]

Summary

• register managed token exchange through framework post-register hooks instead of wiring it directly in main startup
• exchange and persist the manager API token through authenticated manager requests while keeping a single CONFIGS_MANAGER_ACCESS_TOKEN source
• rely on the current instance endpoint selection and generated config defaults for managed startup
• update agent release notes for the managed bootstrap flow changes

Testing

• GO111MODULE=off go test ./...