Change --weight-path argument from required to optional in both serv and run
subcommands. When not specified, it defaults to ~/.aha/{model_id}. Add
get_default_weight_path function to handle the default path resolution.
Update help text to reflect the optional nature of the weight path parameter.
feat(cli): add direct model inference via new run subcommand
- Add `aha run` CLI subcommand for direct model inference without HTTP service
- Support multiple models including Qwen series, OCR models, ASR models, and voice generation
- Implement input/output handling with file path support and auto-generation
- Add comprehensive documentation in CLI_USAGE.md with examples
- Include performance timing for model loading and inference operations
- Add macOS build target to Makefile with Metal support
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