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/modes/interactive/interactive-mode.ts

@@ -525,6 +525,14 @@ export class InteractiveMode {
 
 				return { cancelled: false };
 			},
+			setModelHandler: async (model) => {
+				const key = await this.session.modelRegistry.getApiKey(model);
+				if (!key) return false;
+				await this.session.setModel(model);
+				return true;
+			},
+			getThinkingLevelHandler: () => this.session.thinkingLevel,
+			setThinkingLevelHandler: (level) => this.session.setThinkingLevel(level),
 			isIdle: () => !this.session.isStreaming,
 			waitForIdle: () => this.session.agent.waitForIdle(),
 			abort: () => {