List a document's registered Operating Memory sources

Returns the sources registered onto the Operating Memory subject node projected for one Rhythms document, as a flat list: the authoring chat thread(s), the connected resources (e.g. Linear, Notion, meetings, Slack) that chat read, and discovered related containers. Each row carries the provider, a link, a title, and discovered_at -- set when the system matched the container to the document by topic, null when it was registered as an explicit authoring or read source. No fetched content. First-party only; the curated MCP surface uses /operating_memory/node_sources.