Placeholder BIP-44 coin type needs SLIP-0044 registration

The BIP-44 coin type uses a placeholder value that should be registered with SLIP-0044 for production.

**Location:**
https://github.com/decipherhub/cipherbft/blob/main/crates/crypto/src/mnemonic/derive.rs#L16

```rust
/// Uses 8888 as a placeholder. In production, this should be registered
/// with SLIP-0044 if the project becomes widely adopted.
pub const CIPHERBFT_COIN_TYPE: u32 = 8888;
```