Formal derived_from relation type for memory provenance

[arreyder]

Problem

When a consolidation/summarization process produces a new memory from several source observations (see #5), we need to know which sources it came from — for citation, debugging bad summaries, and cascading deletes.

Today relate_memories supports arbitrary bidirectional links, but there's no standard relation type for provenance. The broker's MemoryPacketItem.Reason is a freeform string, not a queryable edge.

Proposal

Add a reserved relation type derived_from (and its inverse source_of) to relate_memories. Extend list/search/similar tools to optionally follow/include provenance edges.

Schema

No new Solr schema — relations are already stored. Just a convention + validation.

Tooling

• relate_memories rejects unknown relation types when strict=true (keeps freeform as default for back-compat)
• get_memory(id, include_sources=true) returns source observations inline
• delete_memory(id, cascade_derived=true) optionally deletes children

When we'd write derived_from

• Consolidation job produces an extracted_fact from N observation memories
• Bulk import of a conversation thread → per-turn observations, then a summary memory linked back
• Manual /remember that summarizes a previous scratchpad note

Acceptance

• Documented relation type taxonomy in README
• get_memory supports include_sources
• Validation + unit tests
• Consolidation issue (Memory consolidation/summarization tool #5) uses this when implemented

Prior art

agentmemory calls this "citation provenance" — every compressed memory traces back to source observations.