feat: add xhigh thinking level support for gpt-5.2 and gpt-5.2-codex

- Add XHIGH_MODELS constant and getAvailableThinkingLevels() to AgentSession
- Update ThinkingSelectorComponent to accept availableLevels parameter
- Both shift+tab cycling and /thinking command now show xhigh for supported models
- Update types.ts documentation to list supported models

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -1301,6 +1301,7 @@ export class InteractiveMode {
 		this.showSelector((done) => {
 			const selector = new ThinkingSelectorComponent(
 				this.session.thinkingLevel,
+				this.session.getAvailableThinkingLevels(),
 				(level) => {
 					this.session.setThinkingLevel(level);
 					this.updateEditorBorderColor();