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. Phase 2e moved every piece of view state (composer, popup
chrome, render caches, theme) into the TUI crate's AppView.
Doc-lint allows: the domain surface the TUI/CLI/host drive directly
(Db, provider, config, App's event handlers) is still pub, so the
per-item doc lints would be noise until the Phase 4 host API pass
privatizes it. They're crate-scoped deliberately — the 2e goal (zero
TUI deps) is unaffected.