Add a source adapter

Use a source adapter when a provider or local input surface needs to publish a new Rainrail event.

Steps

1. Define the provider input shape and the normalized event names it can emit.
2. Map provider delivery metadata into delivery, occurredAt, source, and subject.
3. Put workflow-needed facts in payload and keep raw provider data behind rawPayload references or redacted markers.
4. Generate deterministic event IDs for retries and duplicate deliveries.
5. Add focused tests for normalization, redaction, credential-looking values, and edge-case provider payloads.
6. Update the source spec and contracts manifest if public exports or contract docs change.

Source spec

Adapter boundaries are defined in docs/core-eep-bridge-source-adapter-boundary.md.