fix(eval): satisfy Rust 1.98 clippy

[ECD5A]

Summary

Rust 1.98 added the clippy::useless_format lint, causing the eval-tool Clippy job to fail on three constant marker strings. Use .to_string() for those constants without changing runtime behavior.

Scope

• Core storage / recall
• CLI / TUI
• MCP / SDK
• Security
• Release / packaging
• Documentation

Safety Checklist

• I did not change the storage layout without a design rationale.
• I did not change recall semantics without tests.
• I did not add a new filter family without benchmark evidence.
• I did not use JSON/JSONL as runtime storage.
• I did not commit generated stores, target artifacts, passphrases, logs, or private corpus data.

Verification

• cargo fmt --check
• cargo test
• cargo check -p mge-cli --bins
• Release / smoke checks if packaging or integration changed
• cargo +1.98.0 clippy --manifest-path tools/agent-memory-eval/Cargo.toml --all-targets -- -D warnings

Notes

Release checks were not run because packaging and integration behavior are unchanged.