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/coding-agent/src/core/extensions/index.ts

@@ -11,6 +11,8 @@ export type {
 	// Re-exports
 	AgentToolResult,
 	AgentToolUpdateCallback,
+	// App keybindings (for custom editors)
+	AppAction,
 	AppendEntryHandler,
 	BashToolResultEvent,
 	BeforeAgentStartEvent,
@@ -42,6 +44,7 @@ export type {
 	GetAllToolsHandler,
 	GetThinkingLevelHandler,
 	GrepToolResultEvent,
+	KeybindingsManager,
 	LoadExtensionsResult,
 	// Loaded Extension
 	LoadedExtension,