Polish CodexUI thread, inspector, and conversation UX

[VolkerChristian]

Summary

• add explicit thread sorting, card styling, canonical menus, palettes, typography, and primary panel headers
• improve inspector navigation and Git-backed multi-repository change review with libgit2
• make command execution cards content-sized, scrollable, bottom-following, and stable during streaming
• retain the shared composer draft across thread navigation and keep nested output overscroll inside its card
• update canonical UI behavior and design documentation

Validation

• incremental Debug build completed successfully
• full CTest suite: 7/7 passed
• live visual verification covered command sizing, delayed streaming, capped output, follow-latest, and prompt animation handoff