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

harness-app:
- EngineHandle: non-blocking multi-turn API (prompt/abort/permission_reply/
  list_sessions/session_messages/message_parts), persistent SQLite store,
  no auto-approve — TUI handles permission asks via real oneshot path
- App refactored to wrap EngineHandle; headless run -p keeps auto-approve

harness-tui:
- Terminal guard (raw mode, alternate screen, panic hook, Drop restore)
- Event loop: tokio::select! over crossterm events, bus events, 33ms render tick
- AppState with MessageView/PartView (cached Vec<Line> field), ModalState
- pulldown-cmark → ratatui markdown renderer (headings, bold, italic, code
  blocks, lists, blockquotes, links, manual word-wrap)
- Layout: header, chat viewport, input (tui-textarea), status bar
- Permission modal (y/a/n) wired to EngineHandle::permission_reply
- Session picker (Ctrl+S) with resume
- Abort (Esc), Ctrl+C×2 quit, slash commands (/new /model /agent /sessions)
- 7 TestBackend snapshot tests (empty, messages, tool cards, modals)

crates/harness-tui/Cargo.toml

@@ -12,6 +12,21 @@ path = "src/main.rs"
 harness-app = { workspace = true }
 harness-core = { workspace = true }
 tokio = { workspace = true }
+ratatui = { workspace = true }
+crossterm = { workspace = true }
+tui-textarea = { workspace = true }
+pulldown-cmark = { workspace = true }
+tracing = { workspace = true }
+tracing-subscriber = { workspace = true }
+tracing-appender = { workspace = true }
+dirs = { workspace = true }
+anyhow = { workspace = true }
+serde_json = { workspace = true }
+futures = { workspace = true }
+tokio-util = { workspace = true }
+
+[dev-dependencies]
+insta = "1"
 
 [lints]
 workspace = true