.NET: Add skill to verify samples

dotnet/.github/skills/verify-dotnet-samples/SKILL.md

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+---
+name: verify-dotnet-samples
+description: > How to build, run and verify the .NET sample projects in the Agent Framework repository. Use this when a user wants to verify that the samples still function as expected.
+---
+
+# Verifying .NET Sample Projects
+
+## Sample Pre-requisites
+
+We should only support verifying samples that:
+1. Use environment variables for configuration.
+2. Have no complex setup requirements, e.g., where multiple applications need to be run together, or where we need to launch a browser, etc.
+
+Always report to the user which samples were run and which were not, and why.
+
+## Verifying a sample
+
+Samples should be verified to ensure that they actually work as intended and that their output matches what is expected.
+For each sample that is run, output should be produced that shows the result and explains the reasoning about what output
+was expected, what was produced, and why it didn't match what the sample was expected to produce.
+
+Steps to verify a sample:
+1. Read the code for the sample
+1. Check what environment variables are required for the sample
+1. Check if each environment variable has been set
+1. If there are any missing, give the user a list of missing environment variables to set and terminate
+1. Summarize what the expected output of the sample should be
+1. Run the sample
+1. Show the user any output from the sample run as it gets produced, so that they can see the run progress
+1. Check the output of the run against expectations
+1. After running all requested samples, produce output for each sample that was verified:
+  1. If expectations were matched, output the following:
+     ```text
+     [Sample Name] Succeeded
+     ```
+  1. If expectations were not matched, output the following:
+     ```text
+     [Sample Name] Failed
+     Actual Output:
+     [What the sample produced]
+     Expected Output:
+     [Explanation of what was expected and why the actual output didn't match expectations]
+     ```
+
+## Environment Variables
+
+Most samples use environment variables to configure settings.
+
+```csharp
+var endpoint = Environment.GetEnvironmentVariable("AZURE_OPENAI_ENDPOINT") ?? throw new InvalidOperationException("AZURE_OPENAI_ENDPOINT is not set.");
+var deploymentName = Environment.GetEnvironmentVariable("AZURE_OPENAI_DEPLOYMENT_NAME") ?? "gpt-4o-mini";
+```
+
+To run a sample, the environment variables should be set first.
+Before running a sample, check whether each environment variable in the sample has a value and
+then give the user a list of environment variables to set.
+
+You can provide the user some examples of how to set the variables like this:
+
+```bash
+export AZURE_OPENAI_ENDPOINT="https://my-openai-instance.openai.azure.com/"
+export AZURE_OPENAI_DEPLOYMENT_NAME="gpt-4o-mini"
+```
+
+To check if a variable has a value use e.g.:
+
+```bash
+echo $AZURE_OPENAI_ENDPOINT
+```
+
+## How to Run a Sample (General Pattern)
+
+```bash
+cd dotnet/samples/<category>/<sample-dir>
+dotnet run
+```
+
+For multi-targeted projects (e.g., Durable console apps), specify the framework:
+
+```bash
+dotnet run --framework net10.0
+```