Add AI instructions file and update wiki submodules with unicode fixes

.ai-instructions.md

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+# AI Assistant Instructions for 3D Print Meetup Repository
+
+## Important Guidelines for AI Assistants
+
+### Character Usage Rules
+
+**NEVER use the unicode multiplication symbol (×) in any documentation.**
+
+- ❌ WRONG: `25mm × 25mm × 25mm`
+- ✅ CORRECT: `25mm x 25mm x 25mm`
+
+**Always use lowercase 'x' for dimensions, not the unicode × (U+00D7) character.**
+
+This applies to:
+
+- Challenge documentation
+- Wiki pages
+- README files
+- Any markdown documentation
+- Announcements
+- All text content
+
+### Reason
+
+The unicode × character can cause display issues in:
+
+- Email clients
+- Reddit posts
+- Various text editors
+- Screen readers
+- Plain text environments
+
+Using lowercase 'x' ensures universal compatibility across all platforms where this content will be shared.
+
+### Quick Check
+
+Before committing any files, search for the × character and replace with lowercase x.
+
+PowerShell command to check:
+
+```powershell
+Get-ChildItem -Recurse -Include *.md | Select-String "×"
+```
+
+---
+
+**Last Updated**: October 28, 2025