Add audit log with tamper detection

Maintain a log of all actions taken within the password manager, with cryptographic integrity checks to detect tampering.

The issue names no files, tests, or entry points. First identify where actions are handled and how persistent data is stored, then define the audit record and integrity-check requirements. Done means all password-manager actions are logged and tampering can be detected, with tests covering those guarantees.

Written by the indexing model from the issue text.