Add error handling for missing API keys on startup

[thegreatalxx]

Summary

When Conductor starts up with a missing or invalid API key (e.g. for Claude, OpenAI, Gemini), the error message can be cryptic or buried in a stack trace. New users frequently get stuck at this step. A clear, friendly error at startup would significantly improve the onboarding experience.

What to build

In src/core/Conductor.ts (or wherever the AI provider is initialized), add a check that:

1. Detects when the configured API key is missing or empty
2. 2. Prints a clear, human-readable error message explaining what's wrong
3. 3. Points the user to the fix: conductor ai setup
      Example of what it should look like:

✗ Missing API key for Claude.
  Run: conductor ai setup → choose your provider and enter your key.
  Or set ANTHROPIC_API_KEY in your environment.

Where to look

• src/core/Conductor.ts — main initialization
• • src/ai/ — provider implementations (ClaudeProvider, OpenAIProvider, etc.)
• • • Each provider's test() method already validates the connection — the startup check could call test() early and catch failures gracefully

Why it's a good first issue

• Isolated change — only touches startup/init logic
• • No new dependencies needed
• • • Improves DX for every new user of the project
• • • • Good way to learn how the AI provider layer works

Acceptance criteria

• Starting conductor with no API key shows a clear error message (not a raw stack trace)
• • Message includes the command to fix it (conductor ai setup)
• • • Works for all supported providers (Claude, OpenAI, Gemini, Ollama, OpenRouter)