refactor: move generated_image out of tui to break agent->tui edge

generated_image only depended on side_panel (out of cycle) and the
already-extracted jcode-terminal-image crate (via the tui::image_metadata
re-export). Promote it to a top-level module so agent (and others) no longer
reach into tui for it. tui keeps backward-compat re-exports.

- git mv src/tui/generated_image.rs -> src/generated_image.rs
- internal crate::tui::image_metadata:: -> jcode_terminal_image::metadata::
- agent/utils.rs uses crate::generated_image:: directly
- module now reports cycle membership = none (extractable)