Source setup orchestration

The web source-add flow currently owns a lot of business logic that agents have to rediscover through low-level tools: preset lookup, URL-to-endpoint mapping, endpoint probing, OAuth strategy choice, connection id generation, browser handoff, credential binding, and final source registration.

Longer term, consider moving this into an SDK-level source setup service so the frontend and agent tools share the same state machine. The frontend would render steps from the service, while agent tools would return the same state with model-facing instructions fields. Keep low-level plugin tools as escape hatches, but make common preset flows first-class.