Phase 1.3: Implement auth.py

[J-MaFf]

Implement auth.py with multi-fallback authentication chain for all API keys.

Deliverables

• Auth chain: CLI args → env vars → 1Password SDK → 1Password CLI → manual prompts
• ClickUp API key retrieval
• Gemini API key retrieval
• Secure credential masking in logs
• Error handling for missing credentials
• Unit tests for auth module