feat(01-03): implement echo chat UI with MudBlazor

Add ChatGPT-style chat interface using MudBlazor components. User messages
display on the right (purple), bot echoes "success msg!" on the left.
Includes app bar, centered 768px layout, bottom-anchored messages, and
documented inline comments on every Blazor concept introduced.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -1,2 +1,58 @@
-# AgenticCode
+# ChatAgent
 
+A personal AI chat web application built with Blazor WebAssembly and MudBlazor. Currently in the **Echo phase** — the bot responds with "success msg!" to every message.
+
+## Prerequisites
+
+- [.NET 9 SDK](https://dotnet.microsoft.com/download/dotnet/9.0)
+
+## Project Structure
+
+```
+src/
+  ChatAgent.Client/    Blazor WASM app (runs in the browser)
+  ChatAgent.Api/       ASP.NET Core Web API (backend proxy)
+  ChatAgent.Shared/    Shared models between Client and Api
+```
+
+## Run
+
+### Chat UI only (Echo phase)
+
+The echo phase doesn't need the API backend — the client handles everything locally.
+
+```bash
+cd src/ChatAgent.Client
+dotnet run
+```
+
+Open http://localhost:5100 in your browser.
+
+### Full stack (Client + API)
+
+Run both projects in separate terminals:
+
+```bash
+# Terminal 1 — API backend
+cd src/ChatAgent.Api
+dotnet run
+
+# Terminal 2 — Blazor WASM client
+cd src/ChatAgent.Client
+dotnet run
+```
+
+| Service | HTTP | HTTPS |
+|---------|------|-------|
+| Client  | http://localhost:5100 | https://localhost:5200 |
+| API     | http://localhost:7000 | https://localhost:7100 |
+
+The health check page is available at `/health` when the API is running.
+
+## Build
+
+From the repo root:
+
+```bash
+dotnet build
+```