feat: add Tab key to cycle through thinking levels

- Add onTab callback to CustomEditor
- Implement cycleThinkingLevel() in TuiRenderer
- Only works when model supports reasoning
- Cycles through: off → minimal → low → medium → high → off
- Update footer to show current thinking level with '(tab to cycle)' hint
- Update header instructions to mention tab for thinking
- Show notification when thinking level changes

packages/coding-agent/src/tui/custom-editor.ts

@@ -6,8 +6,17 @@ import { Editor } from "@mariozechner/pi-tui";
 export class CustomEditor extends Editor {
 	public onEscape?: () => void;
 	public onCtrlC?: () => void;
+	public onTab?: () => boolean;
 
 	handleInput(data: string): void {
+		// Intercept Tab key when autocomplete is not showing
+		if (data === "\t" && !this.isShowingAutocomplete() && this.onTab) {
+			const handled = this.onTab();
+			if (handled) {
+				return;
+			}
+		}
+
 		// Intercept Escape key - but only if autocomplete is NOT active
 		// (let parent handle escape for autocomplete cancellation)
 		if (data === "\x1b" && this.onEscape && !this.isShowingAutocomplete()) {