Adds harness-core::engine::compact, a Compactor triggered when a session approaches its context limit, and threads the overflow trigger through the engine loop so a session driven past the limit compacts and continues correctly.
Lets subagents report context files back to the job board (surfaced from the read tool) so the parent session's next-turn context includes what a background child has been looking at.
Adds the task tool and subagent spawner in harness-tools, and wires spawning/aliasing/reuse and depth limits through harness-app and the job board, so an orchestrator session can launch and track child sessions.
Adds a bundled models.dev snapshot plus harness-providers::modelsdev to resolve model metadata, and wires live cost display into the TUI's input bar and message state, updating the render snapshots accordingly.
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