docs: add CLAUDE.md for Claude Code guidance

Provides build commands, architecture overview, and development
conventions for AI-assisted development.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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 & Test Commands
+
+```bash
+dotnet build                # Build the solution
+dotnet test                 # Run all tests
+dotnet test --filter "FullyQualifiedName~DocumentToolsTests"  # Run specific test class
+dotnet test --logger "console;verbosity=detailed"             # Verbose test output
+```
+
+**Running locally:**
+```bash
+# Set required environment variables
+export PAPERLESS_BASE_URL=https://your-instance.com
+export PAPERLESS_API_TOKEN=your-token
+
+# stdio mode (for Claude Desktop)
+dotnet run --project PaperlessMCP -- --stdio
+
+# HTTP mode (runs at http://localhost:5000/mcp)
+dotnet run --project PaperlessMCP
+```
+
+## Architecture
+
+PaperlessMCP is an MCP (Model Context Protocol) server that bridges Paperless-ngx with AI assistants. It supports dual transport modes: stdio for Claude Desktop integration and HTTP for remote usage.
+
+### Key Components
+
+- **Program.cs** - Entry point with dual transport setup (stdio vs HTTP). Configures DI, HttpClient with Polly retry policies, and MCP server registration.
+
+- **Client/PaperlessClient.cs** - Central API client for all Paperless-ngx operations. Uses snake_case JSON serialization to match API conventions.
+
+- **Client/PaperlessAuthHandler.cs** - DelegatingHandler that injects API token authentication.
+
+- **Tools/** - MCP tool implementations, one file per entity type:
+  - `DocumentTools.cs` - Search, CRUD, bulk operations, upload (base64 and file path)
+  - `TagTools.cs`, `CorrespondentTools.cs`, `DocumentTypeTools.cs`, `StoragePathTools.cs` - Entity management with matching algorithms
+  - `CustomFieldTools.cs` - Custom field definitions and assignments
+  - `HealthTools.cs` - Ping and capabilities
+
+- **Models/** - Request/response DTOs organized by entity (Documents/, Tags/, etc.). Uses `PaginatedResponse<T>` for list endpoints.
+
+### Configuration
+
+Environment variables (take precedence over appsettings.json):
+- `PAPERLESS_BASE_URL` / `PAPERLESS_URL` - Paperless-ngx instance URL
+- `PAPERLESS_API_TOKEN` / `PAPERLESS_TOKEN` - API authentication token
+- `MAX_PAGE_SIZE` - Max items per page (default: 100)
+- `MCP_PORT` - HTTP server port (default: 5000)
+
+### Testing
+
+Tests use xUnit with FluentAssertions for assertions, NSubstitute for mocking, and RichardSzalay.MockHttp for HTTP mocking. Test files mirror the main project structure under `PaperlessMCP.Tests/`.
+
+## Conventions
+
+- Use [Conventional Commits](https://www.conventionalcommits.org/) - version bumps are automatic based on commit type (`fix:` = patch, `feat:` = minor, `feat!:` = major)
+- Trunk-based development: feature branches merge directly to `main`
+- All destructive operations (delete, bulk operations) require explicit `confirm=true` and default to dry-run mode