Add picloud_shared::crypto: AES-256-GCM encrypt/decrypt envelope
(12-byte CSPRNG nonce, 128-bit tag appended to ciphertext) plus a
MasterKey sourced from PICLOUD_SECRET_KEY (base64 of 32 bytes), with
a deterministic dev-key fallback gated on PICLOUD_DEV_MODE=true. Unset
key without dev mode is fatal. Key rotation is out of v1.1.7 scope.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
dc2e4fa01f