Relocate the AI chat layer (chat_provider.rs, chat_models.rs,
agent_settings_state.rs) out of openpencil-shell-core into a dedicated
op-ai crate. These three modules are dependency-free transport-free
data shapes — the ChatProvider trait, the ModelEntry catalog type, and
the Cmd+, settings-modal state — so they form a clean wasm32-clean
leaf crate. A fresh lib.rs declares all three as modules; chat_models'
`crate::agent_settings_state::` path stays valid. openpencil-desktop's
chat_*.rs transports + model_discovery.rs now import `op_ai::*`. Pure
relocation, no behaviour change.
