XY-801: [ELF vNext] agentmemory import and benchmark adapter boundary

[yvette-carlisle]

Summary

• Add an offline elf-eval adapter for sanitized agentmemory-style session fixtures.
• Emit note candidates, doc candidates, baseline queries, and ignored-item reasons without calling network services or writing ELF storage.
• Add a sample fixture, integration test, and research guide covering mapping, ignored items, and retrieval comparison limits.

Verification

• cargo make fmt
• cargo make lint-fix
• cargo test -p elf-eval --test agentmemory_fixture_adapter
• cargo run -p elf-eval --bin agentmemory_fixture_adapter -- --fixture apps/elf-eval/fixtures/agentmemory/sample_session.json --out tmp/agentmemory-adapter.json
• cargo make checks