Add learning hour: Show, Don't Tell — using examples with coding agents

Exercise project with Order domain and NUnit tests demonstrating
the 'show, don't tell' pattern for guiding coding agents.

- 1 refactored test using custom matcher (the example)
- 5 unrefactored tests with multiple asserts (the practice)
- OrderStateConstraint custom matcher for NUnit 3

exercises/show-dont-tell/.gitignore

@@ -0,0 +1,19 @@
+## NuGet
+*.nupkg
+**/packages/
+
+## Build
+bin/
+obj/
+
+## IDE
+*.user
+*.suo
+*.userosscache
+*.sln.docstates
+.idea/
+*.slnx
+*.slnx launches
+
+## NuGet local config (workaround for locked ~/.nuget)
+nuget.config