fix(tools,agent-context): replace sync fs with tokio in FileExecutor, dedup box_err

[bug-ops]

Summary

• FileExecutor: 6 sync std::fs handlers block tokio worker thread after partial #4502 fix #4507: Replace 6 remaining std::fs blocking call sites in FileExecutor that were missed in the partial fix (fix(tools,index): replace blocking std::fs with tokio::fs and wire embed_provider #4502). Simple handlers (handle_create_directory, handle_delete_path, handle_move_path, handle_copy_path) are converted to tokio::fs equivalents. Recursive helpers (grep_recursive, copy_dir_recursive) are wrapped in tokio::task::spawn_blocking since the traversal loop cannot be trivially made async.
• refactor(agent-context): deduplicate Box<dyn Error> error-boxing pattern in memory_backend.rs #4511: Extract a private box_err helper in memory_backend.rs and replace 12 identical map_err(|e| Box::new(e) as Box<dyn Error + Send + Sync>) closures — no behavior change.

Test plan

• cargo +nightly fmt --check passes
• cargo clippy --workspace -- -D warnings — zero warnings
• cargo nextest run --workspace --lib --bins — 10054 tests passed, 0 failed
• RUSTFLAGS="-D warnings" cargo check --workspace --all-targets --features desktop,ide,server,chat,pdf,scheduler --locked passes
• Reviewer verified all 6 blocking call sites eliminated; spawn_blocking closures satisfy 'static + Send; box_err bounds correct for all 12 call sites

Closes #4507, #4511