risk_index, community_summaries and flow_snapshots have no readers; rows go stale and dangle #1059

Across the package these three tables are only ever written (DELETE + INSERT in tools/build.py::_compute_summaries); no query, tool or CLI command reads them back. Two consequences:

• The work to compute them is paid on every full postprocess and never used.
• remove_files_permanently deletes from nodes, edges and embeddings only, and PRAGMA foreign_keys is never enabled, so risk_index.node_id rows keep pointing at deleted nodes. One graph here carries ~18k dangling risk_index rows; if a reader is ever added it will read garbage.

Either wire them into the tools that would benefit (risk scoring on the read path, cached community summaries) and add them to the deletion path, or drop them from the schema. The current state is the worst of both.

Same class, smaller: flow_memberships.node_id and nodes.community_id also survive remove_files_permanently, but those at least have readers.