feat(core): initial release with 3-tier secret storage

Implements SmartSecret with macOS Keychain, Linux secret-tool, and AES-256-GCM encrypted file fallback backends. Zero runtime dependencies.

ts/smartsecret.backends.base.ts

@@ -0,0 +1,10 @@
+export type TBackendType = 'macos-keychain' | 'linux-secret-service' | 'file-encrypted';
+
+export interface ISecretBackend {
+  readonly backendType: TBackendType;
+  isAvailable(): Promise<boolean>;
+  setSecret(account: string, secret: string): Promise<void>;
+  getSecret(account: string): Promise<string | null>;
+  deleteSecret(account: string): Promise<boolean>;
+  listAccounts(): Promise<string[]>;
+}