feat(coding-agent): add ctx.ui.setEditorComponent() extension API

- Add setEditorComponent() to ctx.ui for custom editor components
- Add CustomEditor base class for extensions (handles app keybindings)
- Add keybindings parameter to ctx.ui.custom() factory (breaking change)
- Add modal-editor.ts example (vim-like modes)
- Add rainbow-editor.ts example (animated text highlighting)
- Update docs: extensions.md, tui.md Pattern 7
- Clean up terminal on TUI render errors

packages/tui/src/editor-component.ts

@@ -0,0 +1,65 @@
+import type { AutocompleteProvider } from "./autocomplete.js";
+import type { Component } from "./tui.js";
+
+/**
+ * Interface for custom editor components.
+ *
+ * This allows extensions to provide their own editor implementation
+ * (e.g., vim mode, emacs mode, custom keybindings) while maintaining
+ * compatibility with the core application.
+ */
+export interface EditorComponent extends Component {
+	// =========================================================================
+	// Core text access (required)
+	// =========================================================================
+
+	/** Get the current text content */
+	getText(): string;
+
+	/** Set the text content */
+	setText(text: string): void;
+
+	// =========================================================================
+	// Callbacks (required)
+	// =========================================================================
+
+	/** Called when user submits (e.g., Enter key) */
+	onSubmit?: (text: string) => void;
+
+	/** Called when text changes */
+	onChange?: (text: string) => void;
+
+	// =========================================================================
+	// History support (optional)
+	// =========================================================================
+
+	/** Add text to history for up/down navigation */
+	addToHistory?(text: string): void;
+
+	// =========================================================================
+	// Advanced text manipulation (optional)
+	// =========================================================================
+
+	/** Insert text at current cursor position */
+	insertTextAtCursor?(text: string): void;
+
+	/**
+	 * Get text with any markers expanded (e.g., paste markers).
+	 * Falls back to getText() if not implemented.
+	 */
+	getExpandedText?(): string;
+
+	// =========================================================================
+	// Autocomplete support (optional)
+	// =========================================================================
+
+	/** Set the autocomplete provider */
+	setAutocompleteProvider?(provider: AutocompleteProvider): void;
+
+	// =========================================================================
+	// Appearance (optional)
+	// =========================================================================
+
+	/** Border color function */
+	borderColor?: (str: string) => string;
+}