M2: TUI — EngineHandle, ratatui app, markdown renderer, permission modal, session picker, snapshot tests

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.

crates/harness-tui/src/modal.rs

@@ -0,0 +1,38 @@
+//! Modal helpers for permission dialogs and the session picker.
+//!
+//! Rendering lives in `render.rs`; key handling lives in `input.rs`. This module
+//! provides shared utilities for modal state transitions.
+
+use harness_app::EngineHandle;
+use harness_core::permission::PermissionReply;
+use harness_core::types::Session;
+
+use crate::state::{AppState, ModalState};
+
+/// Resolve the current permission request with `reply`.
+pub fn resolve_permission(state: &mut AppState, engine: &EngineHandle, reply: PermissionReply) {
+    state.handle_permission_reply(reply, engine);
+}
+
+/// Open the session picker with the supplied sessions.
+pub fn open_session_picker(state: &mut AppState, sessions: Vec<Session>) {
+    state.open_session_picker(sessions);
+}
+
+/// Load a session's messages and parts into state.
+pub async fn select_session(
+    state: &mut AppState,
+    engine: &EngineHandle,
+    session: Session,
+) -> Result<(), harness_app::AppError> {
+    let session_id = session.id.clone();
+    let messages = engine.session_messages(session_id).await?;
+    let mut all_parts = Vec::new();
+    for message in &messages {
+        let parts = engine.message_parts(message.id.clone()).await?;
+        all_parts.extend(parts);
+    }
+    state.set_session(session, messages, all_parts);
+    state.modal = ModalState::None;
+    Ok(())
+}