feat: Add Signal messenger support via signal-cli

Implements Signal Private Messenger integration using the signal-cli
external tool for RustyClaw issue #115.

Changes:
- Add SignalCliMessenger implementation in signal_cli.rs
- Add signal-cli feature flag to Cargo.toml
- Export SignalCliMessenger in messengers/mod.rs
- Add comprehensive documentation and setup guide

Features:
- Send/receive Signal messages through signal-cli process
- Support for media attachments
- Phone number normalization to E.164 format
- Proper error handling and connection management
- Integration tests for core functionality

Prerequisites:
- signal-cli must be installed separately
- Phone number must be registered with Signal
- Configuration in RustyClaw config.toml

Closes #115

Author: rexlunae