fix: handle os.getlogin() OSError in keychain.py

[Dabao21]

Problem

In memory/keychain.py, os.getlogin() is called at module import time to generate the encryption mask:

_MASK = hashlib.sha256(f"{os.getlogin()}@ga_keychain".encode()).digest()

os.getlogin() raises OSError when there is no controlling terminal. This happens in:

• Docker containers
• systemd services
• CI/CD environments (GitHub Actions, etc.)
• Some WSL configurations

When this fails, the entire keychain module cannot be imported, breaking any code that depends on it.

Fix

Catch OSError and fall back to environment variables:

try:
    _user = os.getlogin()
except OSError:
    _user = os.environ.get('USER', os.environ.get('USERNAME', 'default'))
_MASK = hashlib.sha256(f"{_user}@ga_keychain".encode()).digest()

USER is the standard variable on Linux/macOS, USERNAME on Windows. The 'default' fallback ensures the module always loads.

Impact

• keychain module now works in headless/CI environments
• Encryption mask remains deterministic per-user (same user → same mask)
• No behavior change for normal terminal sessions