## Why

The chat backend currently proxies requests to an OpenAI-compatible API (CLIProxyAPI) via manual HttpClient calls and SSE parsing. As the agent evolves toward structured extraction with tool calling and autonomous validation loops, this manual plumbing becomes a liability. Semantic Kernel provides a production-ready abstraction for chat completion, tool/function calling, and auto-invocation — letting us focus on agent behavior rather than HTTP mechanics. Adopting it now establishes the foundation for the agentic workflow (natural language → structured extraction → tool-based validation → human-in-the-loop clarification).

## What Changes

- Replace manual HttpClient + SSE proxy in `ChatController` with Semantic Kernel's `OpenAIChatCompletionService` pointed at the existing CLIProxyAPI proxy
- Add a validation plugin that the agent can call as a tool to validate extracted key-value output against a predefined schema
- Introduce an agentic loop: the kernel autonomously retries extraction up to 2–3 times on validation failure, then escalates to the user for clarification
- Keep the existing SSE contract to the Blazor client unchanged — `ChatApiClient` and `Chat.razor` are not modified
- **BREAKING**: `ChatController` internals are rewritten; the manual Responses API proxy logic is removed entirely

## Capabilities

### New Capabilities
- `agent-extraction`: Defines the structured field extraction behavior — predefined keys, validation rules, autonomous retry loop, and human-in-the-loop escalation
- `semantic-kernel-integration`: Defines how Semantic Kernel is configured, registered, and wired into the API — kernel setup, OpenAI connector config, plugin registration

### Modified Capabilities
- `chat-streaming`: The streaming requirement changes from "proxy SSE from upstream API" to "stream Semantic Kernel chat completion responses as SSE" — same client contract, different server implementation

## Impact

- **ChatAgent.Api**: New NuGet dependencies (`Microsoft.SemanticKernel`), `Program.cs` service registration changes, `ChatController` rewritten
- **ChatAgent.Api.Tests**: Existing `ChatControllerTests` need updating to mock Semantic Kernel services instead of upstream HTTP calls
- **Dependencies**: Adds `Microsoft.SemanticKernel` and `Microsoft.SemanticKernel.Connectors.OpenAI` packages
- **Infrastructure**: No change — still talks to CLIProxyAPI at `localhost:8317`
- **Client**: No change — SSE contract preserved