Add ExtensionAPI methods, preset example, and TUI documentation improvements

- ExtensionAPI: setModel(), getThinkingLevel(), setThinkingLevel() methods
- New preset.ts example with plan/implement presets for model/thinking/tools switching
- Export all UI components from pi-coding-agent for extension use
- docs/tui.md: Common Patterns section with copy-paste code for SelectList, BorderedLoader, SettingsList, setStatus, setWidget, setFooter
- docs/tui.md: Key Rules section for extension UI development
- docs/extensions.md: Exhaustive example links for all ExtensionAPI methods and events
- System prompt now references docs/tui.md for TUI development

Fixes #509, relates to #347

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

@@ -39,6 +39,7 @@ export type {
 	FindToolResultEvent,
 	GetActiveToolsHandler,
 	GetAllToolsHandler,
+	GetThinkingLevelHandler,
 	GrepToolResultEvent,
 	LoadExtensionsResult,
 	// Loaded Extension
@@ -70,6 +71,8 @@ export type {
 	SessionSwitchEvent,
 	SessionTreeEvent,
 	SetActiveToolsHandler,
+	SetModelHandler,
+	SetThinkingLevelHandler,
 	// Events - Tool
 	ToolCallEvent,
 	ToolCallEventResult,