Add modal editing to the agent TUI. (#2)

Adds a status line indicating which mode the user is in.
Adds a "normal" mode with keyboard shortcuts (including a chorded shortcut 'gg').
Adds a command mode with several basic commands that can be entered into an overlay.

Chores:
- Cleans up design/claude/plan.md to avoid confusing claude.
- Adds some TODOs based on claude feedback.`

Reviewed-on: #2
Co-authored-by: Drew Galbraith <drew@tiramisu.one>
Co-committed-by: Drew Galbraith <drew@tiramisu.one>

src/core/orchestrator.rs

@@ -65,6 +65,9 @@ impl<P: ModelProvider> Orchestrator<P> {
         while let Some(action) = self.action_rx.recv().await {
             match action {
                 UserAction::Quit => break,
+                UserAction::ClearHistory => {
+                    self.history.clear();
+                }
 
                 UserAction::SendMessage(text) => {
                     // Push the user message before snapshotting, so providers