Add Signal channel support

## Description

It would be great to add Signal as a communication channel for CoPaw.

## Why Signal?

- **Privacy-focused**: Signal is known for its strong encryption and privacy commitment
- **Growing user base**: Many users prefer Signal for secure messaging
- **Open source**: Aligns well with CoPaw's open-source philosophy

## Use Case

Users could interact with their CoPaw agent through Signal, receiving notifications and sending commands via their preferred secure messaging platform.

## Implementation Notes

Signal has a [libsignal](https://github.com/signalapp/libsignal) library and various community-maintained APIs that could be leveraged for this integration.

---

Would love to contribute to this feature if there's interest!