feat: add on-device persistent memory module

[zhaog100]

On-Device Persistent Memory with Cryptographic Identity Binding

Implements the feature request from #1121.

Features

• Cryptographic Identity Management: PBKDF2-HMAC-SHA256 key derivation with UUIDv4 identity
• AES-256 Encrypted Storage: Local storage of conversation history and user profile
• Context Compression: Tiered memory management with configurable compression ratios
• Privacy-First: Zero data exfiltration - all data stays on device

Modules

Class	Purpose
IdentityManager	Cryptographic identity generation and management
LocalEncryptedStore	AES-256-GCM encrypted local storage
ContextCompressor	Conversation history compression
PersistentMemoryManager	High-level API for client applications

Usage Example

from persistent_memory import PersistentMemoryManager

# Initialize
manager = PersistentMemoryManager()

# Add conversation
manager.add_message("user", "My name is Alice and I live in Tokyo.")
manager.add_message("assistant", "Nice to meet you, Alice!")

# Update user profile
manager.update_key_fact("name", "Alice")
manager.update_key_fact("location", "Tokyo")

# Get system prompt with persistent context
system_prompt = manager.get_system_prompt()

# Export for GDPR compliance
export = manager.export_data()

# Delete all data (forget me)
manager.delete_all_data()

Security Architecture

• Identity key: PBKDF2-HMAC-SHA256 (10000 iterations)
• Storage encryption: AES-256-GCM
• Key storage: Platform keychain (Android Keystore / iOS Keychain) recommended for production

Reference

• Original issue: Feature Request: On-Device Persistent Memory with Cryptographic Identity Binding (Local Storage Architecture) #1121
• Bounty: $265