feat(AIW-5): Add startup validation for required API keys

[toodimes]

Description

Implements startup validation to prevent the application from starting when required API keys are missing, as specified in Linear ticket AIW-5.

Changes:

• Added environment variable validation during application startup for LINEAR_API_KEY and OPENAI_API_KEY
• Application now fails with clear error messages if either key is missing or empty
• Added openai_api_token() function to Config module for OpenAI API key handling
• Enhanced orchestrator error handling for missing API tokens during runtime
• Added comprehensive test coverage for all startup validation scenarios

Checklist

• I self-reviewed the code
• I updated relevant documentation
• Automated tests demonstrate correctness
• I manually tested it
• I reviewed all direct and transitive dependency changes using diff.hex.pm and/or npmdiff.dev

Security Implications

This change enhances security by ensuring the application cannot run without proper API credentials configured. No additional security review needed as this only adds validation without changing existing authentication flows.

How to Test

1. Test missing LINEAR_API_KEY:

   unset LINEAR_API_KEY && mix run --no-halt
   # Should fail with: "Startup failed: LINEAR_API_KEY environment variable is not set"

2. Test missing OPENAI_API_KEY:

   export LINEAR_API_KEY=test_key && unset OPENAI_API_KEY && mix run --no-halt  
   # Should fail with: "Startup failed: OPENAI_API_KEY environment variable is not set"

3. Test successful startup:

   export LINEAR_API_KEY=test_key OPENAI_API_KEY=test_key && mix run --no-halt
   # Should start successfully and show Symphony dashboard

4. Run test suite:

   export LINEAR_API_KEY=test_key OPENAI_API_KEY=test_key && mix test
   # All 202 tests should pass