Add comprehensive integration capabilities to Pop CLI:
- Multi-account support with named profiles
- Webhook management with signature verification
- External PGP key management (import/export/encrypt/decrypt/sign/verify)
- CLI plugin system for extensibility
- Complete documentation in README.md
All compilation errors fixed and build verified CLEAN.
Security review delegated to FRE-5202.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
- Pass nil refresher to NewProtonMailClient at all 5 call sites
- Change TestListMessages_APIError from 401 to 403 (avoids refresh interception)
- Add error content assertion to TestGetMessage_NotFound