Add LICENSE, unit tests, and encrypted messaging

## Summary
No LICENSE, no tests at all. Encrypted messaging is noted as planned but not yet implemented.

## Tasks
- [ ] Add MIT LICENSE file
- [ ] Add unit tests for keygen, balance, and status commands
- [ ] Implement encrypted messaging with PSK ratcheting (currently only plaintext)
- [ ] Add `--json` output flag for scripting/piping
- [ ] Add message history with local SQLite storage