Make API keys optional & add code quality TODOs

[Miyamura80]

Summary

• Make all 5 LLM API keys (OpenAI, Anthropic, Groq, Perplexity, Gemini) optional with None default
• Add runtime validation in llm_api_key() that raises descriptive errors when a missing key is accessed
• Add code quality issues to TODO.md with high/medium priority items

Test plan

• make ci passes (ruff, vulture, ty, import linter)
• make test passes (14 tests)
• Manually verify app starts with only one API key configured

🤖 Generated with Claude Code

[greptile-apps]

Greptile Overview

Greptile Summary

Made all 5 LLM API keys (OpenAI, Anthropic, Groq, Perplexity, Gemini) optional with None defaults, shifting from required environment variables to optional ones with runtime validation. When llm_api_key() is called for a provider whose key is not configured, it raises a descriptive ValueError indicating which environment variable needs to be set.

The PR also documents existing code quality issues in TODO.md, including the validator anti-pattern in the same file (common/global_config.py:188-198) where validators compute values from environment variables rather than validating inputs.

• Made API keys optional to support apps that only need a subset of providers
• Added clear error messages when accessing unconfigured keys at runtime
• Documented 5 high-priority and 3 medium-priority technical debt items

Confidence Score: 4/5

• Safe to merge with one minor syntax issue in error message formatting
• The implementation correctly makes API keys optional and adds proper runtime validation. The logic is sound and the error messages are helpful. One syntax issue was identified in the error message construction. The test plan shows make ci and make test pass, though manual verification of single-key configuration is pending.
• No files require special attention - the syntax issue in common/global_config.py should be addressed

Important Files Changed

Filename	Overview
common/global_config.py	Made 5 LLM API keys optional with None defaults and added runtime validation that raises descriptive errors when missing keys are accessed
TODO.md	Added code quality section with 5 high-priority and 3 medium-priority technical debt items for future refactoring

[greptile-apps]

_{1 file reviewed, 1 comment}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

Error message inconsistency - this uses {provider.upper()}_API_KEY format, but actual env var names vary (e.g. OPENAI_API_KEY not openai_API_KEY). Suggestion uses the correct uppercase format.

Suggested change

	f"API key for provider '{provider}' is not configured. "
	f"Set {provider.upper()}_API_KEY in your .env file."
	f"Set {provider.upper()}_API_KEY in your .env file."

Prompt To Fix With AI

This is a comment left during a code review.
Path: common/global_config.py
Line: 258:259

Comment:
Error message inconsistency - this uses `{provider.upper()}_API_KEY` format, but actual env var names vary (e.g. `OPENAI_API_KEY` not `openai_API_KEY`). Suggestion uses the correct uppercase format.

```suggestion
                    f"Set {provider.upper()}_API_KEY in your .env file."
```

How can I resolve this? If you propose a fix, please make it concise.