fix(addons): scrub memento source env for stdio

[cedric013]

Summary

• remove LEAN_CTX_SECRET_* fallback variables from stdio child environments
• preserve explicitly restored secret_env target values after source cleanup
• add regression coverage for legacy stdio env inheritance

Tests

• CARGO_TARGET_DIR=/tmp/lean-ctx-pr-1250-target cargo test --lib core::addons::env_scrub::tests -- --nocapture
• rustfmt --edition 2024 --check src/core/addons/env_scrub.rs
• git diff --check -- rust/src/core/addons/env_scrub.rs

Fixes #1250