feat: make terminal backends selectable

[cpendery (cpendery)]

Summary

• add the public Backend enum with Alacritty as the default
• wire backend selection through Rust options, session construction, daemon protocol, and CLI flags
• verify --backend ghostty end to end through Ghostty-only blink reporting

Validation

• workspace clippy with all features
• backend tests with and without the ghostty feature
• CLI unit and Ghostty lifecycle tests

Signed-off-by: cpendery cpendery@vt.edu --- _{Stack created with GitHub Stacks CLI • Give Feedback 💬}