Add xhigh thinking level for OpenAI codex-max models

- Add 'xhigh' to ThinkingLevel type in ai and agent packages
- Map xhigh to reasoning_effort: 'max' for OpenAI providers
- Add thinkingXhigh color token to theme schema and built-in themes
- Show xhigh option only when using codex-max models
- Update CHANGELOG for both ai and coding-agent packages

closes #143

packages/coding-agent/src/theme/dark.json

@@ -65,6 +65,9 @@
 		"thinkingMinimal": "#6e6e6e",
 		"thinkingLow": "#5f87af",
 		"thinkingMedium": "#81a2be",
-		"thinkingHigh": "#b294bb"
+		"thinkingHigh": "#b294bb",
+		"thinkingXhigh": "#d183e8",
+
+		"bashMode": "green"
 	}
 }