fix(file-io): offload move copy from event loop

[RerankerGuo]

Summary

Offload MoveStep's potentially large shutil.copyfile operation so a file move does not block unrelated async service work.

Changes

• Run the copy phase with asyncio.to_thread.
• Keep the existing copy -> retarget links -> unlink ordering unchanged.
• Add a regression test with a controlled slow copy that verifies the event loop remains responsive.

Evidence

Before the fix:

FAILED test_move_copy_does_not_block_event_loop
AssertionError: synchronous copyfile blocked the event loop

After the fix:

1 passed, 48 deselected
49 passed

Validation:

pytest tests/unit/test_crud_steps.py -q
pre-commit run --files reme/steps/file_io/move.py tests/unit/test_crud_steps.py

All checks pass.

Scope

This does not change path validation, overwrite behavior, wikilink retargeting, or source cleanup semantics.