feat: implement just-based build system with distinct binary names

Replace Makefile with justfile for better build management:

Build System Changes:
- Add comprehensive justfile with recipes for both implementations
- Configure Go to build as `mail2couch-go`
- Configure Rust to build as `mail2couch-rs` via Cargo.toml
- Add universal build, test, clean, and install recipes
- Include development convenience recipes and utility commands

New Justfile Features:
- `just build` - builds both implementations
- `just install` - installs both to /usr/local/bin
- `just test` - runs tests for both implementations
- `just sizes` - shows binary size comparison
- `just versions` - compares version outputs
- `just --list` - shows all available recipes

Documentation Updates:
- Update CLAUDE.md with justfile usage examples
- Replace make commands with just equivalents
- Add new utility commands (sizes, versions)
- Update IMPLEMENTATION_COMPARISON.md deployment sections

Benefits of just over make:
- Better command-line interface with `--list`
- More intuitive recipe syntax
- Better suited for development workflows
- Cross-platform compatibility
- Built-in help and documentation

Binary Naming:
- Go implementation: `mail2couch-go` (11M)
- Rust implementation: `mail2couch-rs` (8.3M)
- Clear distinction prevents conflicts
- Parallel installation support

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

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

CLAUDE.md

@@ -5,44 +5,112 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 ## 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
+- **Go implementation**: Located in `/go/` directory, builds as `mail2couch-go`
+- **Rust implementation**: Located in `/rust/` directory, builds as `mail2couch-rs` (fully functional, more advanced)
 
 ## Development Commands
 
-### Go Implementation (Primary)
+### Universal Build Commands (Recommended)
+
+```bash
+# Build both implementations
+just build
+
+# Build individual implementations
+just build-go      # Builds go/mail2couch-go
+just build-rust    # Builds rust/target/release/mail2couch-rs
+
+# Build optimized release versions
+just build-release
+
+# Install both binaries to /usr/local/bin
+sudo just install
+
+# Run tests for both implementations
+just test
+
+# Clean all build artifacts
+just clean
+
+# Format and check code
+just fmt
+just check
+
+# Show binary sizes
+just sizes
+
+# Compare versions
+just versions
+
+# List all available recipes
+just --list
+```
+
+### Go Implementation
 
 ```bash
 # Build the application
-cd go && go build -o mail2couch .
+cd go && go build -o mail2couch-go .
 
 # Run the application with automatic config discovery
-cd go && ./mail2couch
+cd go && ./mail2couch-go
 
 # Run with specific config file
-cd go && ./mail2couch --config /path/to/config.json
+cd go && ./mail2couch-go --config /path/to/config.json
 
 # Run with message limit (useful for large mailboxes)
-cd go && ./mail2couch --max-messages 100
+cd go && ./mail2couch-go --max-messages 100
 
-# Run with both config and message limit
-cd go && ./mail2couch --config /path/to/config.json --max-messages 50
+# Run with dry-run mode
+cd go && ./mail2couch-go --dry-run
 
 # Run linting/static analysis
 cd go && go vet ./...
 
+# Run unit tests
+cd go && go test ./...
+
+# Check dependencies
+cd go && go mod tidy
+```
+
+### Rust Implementation
+
+```bash
+# Build the application (release mode recommended)
+cd rust && cargo build --release
+
+# Run the application with automatic config discovery
+cd rust && ./target/release/mail2couch-rs
+
+# Run with specific config file
+cd rust && ./target/release/mail2couch-rs --config /path/to/config.json
+
+# Run with message limit
+cd rust && ./target/release/mail2couch-rs --max-messages 100
+
+# Run with dry-run mode
+cd rust && ./target/release/mail2couch-rs --dry-run
+
+# Run linting/static analysis
+cd rust && cargo clippy -- -D warnings
+
+# Run unit tests
+cd rust && cargo test
+
+# Format code
+cd rust && cargo fmt
+```
+
+### Integration Testing
+
+```bash
 # Run integration tests with Podman containers
 cd test && ./run-tests.sh
 
 # Run specialized tests
 cd test && ./test-wildcard-patterns.sh
 cd test && ./test-incremental-sync.sh
-
-# Run unit tests (none currently implemented)
-cd go && go test ./...
-
-# Check dependencies
-cd go && go mod tidy
 ```
 
 ## Architecture
@@ -90,11 +158,11 @@ This design ensures the same `config.json` format will work for both Go and Rust
 
 ### Current Implementation Status
 
+#### Both Implementations
 - ✅ Configuration loading with automatic file discovery
-- ✅ Command line flag support for config file path
+- ✅ Command line flag support (--config/-c, --max-messages/-m, --dry-run/-n)
 - ✅ Per-account CouchDB database creation and management
 - ✅ IMAP connection and mailbox listing
-- ✅ Build error fixes
 - ✅ Real IMAP message retrieval and parsing
 - ✅ Email storage to CouchDB framework with native attachments
 - ✅ Folder filtering logic with wildcard support (`*`, `?`, `[abc]` patterns)
@@ -104,15 +172,40 @@ This design ensures the same `config.json` format will work for both Go and Rust
 - ✅ Sync vs Archive mode implementation
 - ✅ CouchDB attachment storage for email attachments
 - ✅ Full message body and attachment handling with MIME multipart support
-- ✅ Command line argument support (GNU-style --max-messages/-m and --config/-c flags)
 - ✅ Per-account CouchDB databases for better organization
 - ✅ Incremental sync functionality with IMAP SEARCH and sync metadata tracking
-- ❌ Rust implementation
+- ✅ Comprehensive --dry-run mode for safe configuration testing
+
+#### Rust Implementation Additional Features
+- ✅ Asynchronous processing with concurrent network operations
+- ✅ Server-side IMAP keyword filtering (more efficient)
+- ✅ Automatic retry logic with exponential backoff
+- ✅ Structured error handling with detailed context
+- ✅ Enhanced CLI with rich help system
+- ✅ Comprehensive unit test coverage
+- ✅ Emoji-enhanced logging for better user experience
+
+#### Go Implementation Characteristics
+- ✅ Sequential processing (simple and reliable)
+- ✅ Minimal dependencies and fast compilation
+- ✅ Client-side keyword filtering
+- ✅ Basic error handling with continue-on-error semantics
 
 ### Key Dependencies
 
+#### Go Implementation
 - `github.com/emersion/go-imap/v2`: IMAP client library
 - `github.com/go-kivik/kivik/v4`: CouchDB client library
+- `github.com/spf13/pflag`: GNU-style command line flags
+- `github.com/emersion/go-message`: Email parsing
+
+#### Rust Implementation
+- `async-imap`: Asynchronous IMAP client
+- `reqwest`: HTTP client for CouchDB API
+- `tokio`: Async runtime and utilities
+- `clap`: Command line argument parsing
+- `serde`: Serialization framework
+- `mail-parser`: Email parsing with MIME support
 
 ### Incremental Sync Implementation