Secret storage for DeepSeek API keys.
Provides a small abstraction (KeyringStore) plus a default
implementation backed by the OS keyring (DefaultKeyringStore),
a file-based fallback for headless Linux (FileKeyringStore), and
an in-memory store for tests (InMemoryKeyringStore).
Higher-level lookup through [Secrets::resolve] checks the keyring first
and falls back to environment variables. Config-file precedence lives in the
config crate so user-facing commands can keep config -> keyring -> env
explicit at the call site.