[Core] Implement secure credential storage using Expo SecureStore

[coderabbitai]

Overview

Build credential management system with hardware-backed secure storage.

Current State

• expo-secure-store dependency installed but unused
• Credential types fully defined in src/types/index.ts
• No implementation exists

Requirements

• Create src/services/credentials/ directory
• Implement CredentialService class:
  • store(type, data) - encrypted storage
  • retrieve(type) - decrypted retrieval
  • validate(type) - check validity
  • delete(type) - secure deletion
• Support all credential types:
  • GitHub tokens (with scopes)
  • Anthropic API keys
  • OpenAI API keys
  • MCP server credentials
• Add token validation against APIs
• Implement secure key derivation
• Add biometric authentication option
• Write security documentation
• Add comprehensive tests

Success Criteria

• All credentials stored in hardware-backed secure enclave
• Zero plaintext credentials in app memory
• Biometric unlock option working
• Security audit passed

Priority: CRITICAL

Estimated Effort: 3-4 days

Labels: core, security, priority:critical

Backlink: Related to PR #2