Adds harness-providers::auth, a persisted auth.json credential store for provider tokens, laying the groundwork for Copilot's device-flow login and token refresh.
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.
Adds the Anthropic SSE codec (codec/anthropic.rs) translating Anthropic's event stream into LlmEvents, the AnthropicProvider, and registry wiring so the engine loop can run against the real API instead of just MockProvider.
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