docs: add CLAUDE.md for Claude Code guidance

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Build Commands
+
+```bash
+# Build the project
+dotnet build EmailSearch/EmailSearch.csproj
+
+# Build release version
+dotnet build EmailSearch/EmailSearch.csproj -c Release
+
+# Run the MCP server (connects via stdio)
+dotnet run --project EmailSearch/EmailSearch.csproj
+```
+
+## Architecture
+
+This is an MCP (Model Context Protocol) server that provides Outlook email search capabilities to LLM clients. It runs as a stdio-based server using the Microsoft.Extensions.Hosting pattern.
+
+**Key Components:**
+
+- `Program.cs` - Entry point that configures the MCP server with stdio transport and registers `EmailSearchTools`
+- `EmailSearchTools.cs` - MCP tool implementations decorated with `[McpServerTool]`:
+  - `SearchEmails` - Search emails with filters (keywords, sender, subject, date range, folder, attachments, importance, category, flag status)
+  - `ReadEmail` - Retrieve full email body by subject and date
+- `SearchFilters.cs` - Filter parameter container for email searches
+- `EmailResult.cs` - DTO for search results with factory method `FromMailItem()`
+
+**Dependencies:**
+
+- `ModelContextProtocol` - MCP SDK for .NET
+- `NetOfficeFw.Outlook` - COM interop wrapper for Outlook automation
+
+**Platform:** Windows-only (.NET 9.0-windows) due to Outlook COM dependency