nexus-chat-core (lib crate nexus_core): the engine behind the nexus
TUI and (later) the nexus host API. Owns all domain logic — sessions, research pipeline, provider
clients, tools, files, skills, SQLite state — with no knowledge of the
terminal UI.
During Phase 2 (workspace split) the App struct still carries view
state and a few TUI-typed fields (composer, theme, render caches);
step 2e moves those into the TUI crate.
Interim lint allows (removed in 2c/2e): the whole App surface is pub
so the TUI crate can drive it directly. Once commands + accessors land,
these items go private again and the doc lints re-apply.