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/examples/extensions/README.md

@@ -36,6 +36,7 @@ cp permission-gate.ts ~/.pi/agent/extensions/
 
 | Extension | Description |
 |-----------|-------------|
+| `preset.ts` | Named presets for model, thinking level, tools, and instructions via `--preset` flag and `/preset` command |
 | `plan-mode.ts` | Claude Code-style plan mode for read-only exploration with `/plan` command |
 | `tools.ts` | Interactive `/tools` command to enable/disable tools with session persistence |
 | `handoff.ts` | Transfer context to a new focused session via `/handoff <goal>` |