docs: sync to Claude Code v2.1.131 (3 patches since v2.1.126)

Sync covers v2.1.128, v2.1.129, and v2.1.131 (May 2026). Highlights:
gateway /v1/models discovery is now opt-in (CLAUDE_CODE_ENABLE_GATEWAY_MODEL_DISCOVERY),
added --plugin-url flag, disableRemoteControl setting, /mcp tool-count display,
expanded skillOverrides values, /context viz no longer leaks into conversation,
Ctrl+R cross-project default. Also reconciles slash-command count to 60+ and
fixes the skill-count entry in CATALOG.

09-advanced-features/planning-mode-examples.md

@@ -533,4 +533,7 @@ Proceed with this plan? (yes/no/modify)
 7. **Plan for testing** at each phase
 
 ---
-**Last Updated**: April 9, 2026
+**Last Updated**: May 6, 2026
+**Claude Code Version**: 2.1.131
+**Sources**:
+- https://code.claude.com/docs/en/changelog