.NET Agent Prompt Templates and Versioning

Current Behavior: Prompts are hardcoded strings in agent creation. No built-in template support or versioning.

Problem: Production systems need to:

Test different prompt variations
Version prompts separately from code
A/B test agent behaviors
Centralize prompt management
Recommended Changes:

Add PromptTemplate abstraction
Support variable substitution
Enable external prompt management (files, config, database)
Provide prompt versioning and rollback
Example Desired API:

```
// Define template
var template = PromptTemplate.FromFile("prompts/analyst-v2.txt");

// Create agent with template
var agent = chatClient.CreateAIAgent(
name: "Analyst",
promptTemplate: template,
tools: tools);

// Runtime variable substitution
var result = await agent.RunAsync(
template.Render(new { childId = "child-123", status = "Nice" }));
```

Start by locating the .NET agent-creation path behind CreateAIAgent and the existing hardcoded prompt handling. Review how prompt inputs are currently passed to agents, then define the scope for templates, variable substitution, external sources, versioning, and rollback. Done should include an agreed API and coverage for the requested prompt-management behavior.

Written by the indexing model from the issue text.