feat: implement Go-based mail2couch with working IMAP and CouchDB integration

- Add configuration system with automatic file discovery (current dir, config subdir, user home, XDG config)
- Implement IMAP client with TLS connection, authentication, and mailbox listing
- Add CouchDB integration with database creation and document storage
- Support folder filtering (include/exclude) and date filtering (since parameter)
- Include duplicate detection to prevent re-storing existing messages
- Add comprehensive error handling and logging throughout
- Structure code in clean packages: config, mail, couch
- Application currently uses placeholder messages to test the storage pipeline
- Ready for real IMAP message parsing implementation

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

Co-Authored-By: Claude <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.
+
+## Project Overview
+
+mail2couch is a utility for backing up mail from various sources (primarily IMAP) to CouchDB. The project supports two implementations:
+- **Go implementation**: Located in `/go/` directory (currently the active implementation)
+- **Rust implementation**: Planned but not yet implemented
+
+## Development Commands
+
+### Go Implementation (Primary)
+
+```bash
+# Build the application
+cd go && go build -o mail2couch .
+
+# Run the application with automatic config discovery
+cd go && ./mail2couch
+
+# Run with specific config file
+cd go && ./mail2couch -config /path/to/config.json
+
+# Run linting/static analysis
+cd go && go vet ./...
+
+# Run tests (currently no tests exist)
+cd go && go test ./...
+
+# Check dependencies
+cd go && go mod tidy
+```
+
+## Architecture
+
+### Core Components
+
+1. **Configuration (`config/`)**: JSON-based configuration system
+   - Supports multiple mail sources with filtering options
+   - CouchDB connection settings
+   - Each source can have folder and message filters
+
+2. **Mail Handling (`mail/`)**: IMAP client implementation
+   - Uses `github.com/emersion/go-imap/v2` for IMAP operations
+   - Supports TLS connections
+   - Currently only lists mailboxes (backup functionality not yet implemented)
+
+3. **CouchDB Integration (`couch/`)**: Database operations
+   - Uses `github.com/go-kivik/kivik/v4` as CouchDB driver
+   - Handles database creation and document management
+   - Defines `MailDocument` structure for email storage
+
+### Configuration Structure
+
+The application uses `config.json` for configuration with the following structure:
+- `couchDb`: Database connection settings (URL, credentials, database name)
+- `mailSources`: Array of mail sources with individual settings:
+  - Protocol support (currently only IMAP)
+  - Connection details (host, port, credentials)
+  - Filtering options for folders and messages
+  - Enable/disable per source
+
+### Configuration File Discovery
+
+The application automatically searches for configuration files in the following order:
+1. Path specified by `-config` command line flag
+2. `./config.json` (current working directory)
+3. `./config/config.json` (config subdirectory)
+4. `~/.config/mail2couch/config.json` (user XDG config directory)
+5. `~/.mail2couch.json` (user home directory)
+
+This design ensures the same `config.json` format will work for both Go and Rust implementations.
+
+### Current Implementation Status
+
+- ✅ Configuration loading with automatic file discovery
+- ✅ Command line flag support for config file path
+- ✅ CouchDB client initialization and database creation
+- ✅ IMAP connection and mailbox listing
+- ✅ Build error fixes
+- ✅ Email message retrieval framework (with placeholder data)
+- ✅ Email storage to CouchDB framework
+- ✅ Folder filtering logic
+- ✅ Date filtering support
+- ✅ Duplicate detection and prevention
+- ❌ Real IMAP message parsing (currently uses placeholder data)
+- ❌ Full message body and attachment handling
+- ❌ Incremental sync functionality
+- ❌ Rust implementation
+
+### Key Dependencies
+
+- `github.com/emersion/go-imap/v2`: IMAP client library
+- `github.com/go-kivik/kivik/v4`: CouchDB client library
+
+### Development Notes
+
+- The main entry point is `main.go` which orchestrates the configuration loading, CouchDB setup, and mail source processing
+- Each mail source is processed sequentially with proper error handling
+- The application currently uses placeholder message data for testing the storage pipeline
+- Message filtering by folder (include/exclude) and date (since) is implemented
+- Duplicate detection prevents re-storing existing messages
+- No tests are currently implemented
+- The application uses automatic config file discovery as documented above
+
+### Next Steps
+
+To complete the implementation, the following items need to be addressed:
+
+1. **Real IMAP Message Parsing**: Replace placeholder message generation with actual IMAP message fetching and parsing using the correct go-imap/v2 API
+2. **Message Body Extraction**: Implement proper text/plain and text/html body extraction from multipart messages
+3. **Attachment Handling**: Add support for email attachments (optional)
+4. **Error Recovery**: Add retry logic for network failures and partial sync recovery
+5. **Performance**: Add batch operations for better CouchDB insertion performance
+6. **Testing**: Add unit tests for all major components
+
+## Development Guidelines
+
+### Code Quality and Standards
+- All code requires perfect linting and tool-formatting, exceptions are allowed only if documented properly