Verifies that getLongTermPrivateKey reads the derivation index from
vault metadata instead of using hardcoded index 0. Test creates a
mock vault with DerivationIndex=5 and confirms the derived key
matches index 5.
Previously, getLongTermPrivateKey() always used derivation index 0 when
deriving the long-term key from a mnemonic. This caused wrong key
derivation for vaults with index > 0 (second+ vault from same mnemonic),
leading to silent data corruption in keychain unlocker creation.
Now reads the vault's actual DerivationIndex from vault-metadata.json.
clawbot