bug: check --no-gpu flag silently ignored for GGUF models

[noahgift]

Bug

In crates/apr-cli/src/commands/check.rs:

1. run_real_checks() (line 134) accepts no_gpu but only passes it to run_real_checks_gguf() — APR and SafeTensors handlers don't receive it (lines 138-140)
2. run_real_checks_gguf() (line 368) accepts _no_gpu but never uses it (underscore prefix) — GPU is always attempted regardless of the flag

Impact

Users running apr check model.gguf --no-gpu expect CPU-only validation, but the flag is silently ignored and GPU may still be used.

Fix

Add warning that --no-gpu is not yet implemented for check command validation.