Adds both OpenAI codecs — chat completions and the newer responses API — plus the OpenAiProvider and registry wiring, so the same session can run against OpenAI in addition to Anthropic.
Builds the ratatui TUI: EngineHandle bridging the async engine to the render loop, chat viewport rendering with a markdown renderer, input handling, a permission modal wired to the real oneshot ask path, and a session picker. Adds TestBackend snapshot tests covering empty session, tool cards (running/completed/error), permission modal, session picker, and message rendering.
Adds the harness-app composition root wiring config, providers, tools, and the engine loop together, plus the harness run -p "<prompt>" debug command in harness-tui::main for driving a one-shot prompt end-to-end from the CLI.
Sets up the Cargo workspace (harness-core, harness-tools, harness-providers, harness-mcp, harness-lsp, harness-app, harness-tui) and the ten architecture docs under docs/. harness-core gets its foundational types (session/message/part/model ids), an in-process event bus, a table-driven permission evaluate function, and a SQLite-backed storage actor with schema and roundtrip coverage. Every crate compiles empty and a bin stub prints its version.
darman