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/light.json

@@ -64,6 +64,9 @@
 		"thinkingMinimal": "#9e9e9e",
 		"thinkingLow": "#5f87af",
 		"thinkingMedium": "#5f8787",
-		"thinkingHigh": "#875f87"
+		"thinkingHigh": "#875f87",
+		"thinkingXhigh": "#8b008b",
+
+		"bashMode": "green"
 	}
 }