docs: clarify extensions only work within patterns, not stdin

- Add prominent warning at top of Extensions guide with visual indicators
- Update main README with brief Extensions section and link to full guide
- Remove misleading examples showing direct piping to fabric
- Add clear examples: ❌ what DOES NOT WORK vs ✅ what WORKS
- Consolidate all extension documentation in Examples/README.md
- Explain technical reason: extensions only processed via ApplyTemplate()
- Prevents user confusion about extension syntax processing

README.md

@@ -705,6 +705,12 @@ Use the `--debug` flag to control runtime logging:
 - `2`: detailed debugging
 - `3`: trace level
 
+### Extensions
+
+Fabric supports extensions that can be called within patterns. See the [Extension Guide](internal/plugins/template/Examples/README.md) for complete documentation.
+
+**Important:** Extensions only work within pattern files, not via direct stdin. See the guide for details and examples.
+
 ## Our approach to prompting
 
 Fabric _Patterns_ are different than most prompts you'll see.