HTTPS support: --tls-cert and --tls-key flags

Add optional `--tls-cert` and `--tls-key` flags for native TLS support.

### Usage

```bash
agentic-memory-mcp serve-http \
  --port 443 \
  --tls-cert /etc/ssl/cert.pem \
  --tls-key /etc/ssl/key.pem \
  --token "secret123"
```

### Alternative

Document nginx/caddy reverse proxy setup as the recommended approach for most deployments (see #7).

Part of #1