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>

rust/Cargo.toml

@@ -65,5 +65,5 @@ name = "mail2couch"
 path = "src/lib.rs"
 
 [[bin]]
-name = "mail2couch"
+name = "mail2couch-rs"
 path = "src/main.rs"