Add documentation: protocol spec, server admin, client guide
docs/PROTOCOL.md (520 lines):
- Identity model (seed → Ed25519 + X25519 via HKDF)
- X3DH key exchange (4 DH operations, ASCII flow diagram)
- Double Ratchet (chain/DH ratchet, skipped keys, state machine)
- KDF chains with domain separation strings
- AEAD (ChaCha20-Poly1305)
- Wire format (WireMessage enum, bincode serialization)
- Pre-key bundle format and lifecycle
docs/SERVER.md (429 lines):
- Build and run instructions
- Full API reference with request/response examples
- Database structure (sled trees)
- Deployment (nginx reverse proxy, systemd unit)
- Security considerations
- Backup and recovery
docs/CLIENT.md (507 lines):
- Quick start guide
- All CLI commands with examples
- Identity management and mnemonic backup
- Web client usage and limitations
- Session and pre-key management
- Threat model table
- Troubleshooting guide
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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