feat: add upsert_session_summary MCP tool for session close-out

New tool batches session discoveries into the knowledge graph:
- Creates a session-audit knowledge entry with summary
- Links it to all touched entities via 'documents' relationships
- Creates individual discovery knowledge entries
- Stamps each entity with last_agent_session attribute
- Updates AGENTS.md with tool listing

AGENTS.md

@@ -112,6 +112,7 @@ elsewhere; regenerate from `internal/mcp/` when tools change):
     get_entity_knowledge(entity_slug) — all docs/investigations/runbooks linked to a slug
     get_knowledge_content(slug) — full markdown body of one knowledge entry
     upsert_knowledge(title, content, about, tags, kind) — write what you learned
+    upsert_session_summary(session_id, summary, entities_touched, discoveries) — batch-write session findings into the graph; creates knowledge entries, links entities, records a session-audit entry
     delete_knowledge(knowledge_slug) — soft-delete a knowledge entry
     restore_knowledge(knowledge_slug) — restore a soft-deleted entry
     merge_knowledge(target_slug, source_slugs) — fold entries into a target