Simplify PageIndex API key config: remove pageindex_api_key_env

[rejojer]

Summary

• Remove pageindex_api_key_env config option, hardcode PAGEINDEX_API_KEY env var directly
• Simplify openkb init by removing one interactive prompt
• Rename pi_api_key to pageindex_api_key for clarity
• Update README and config.yaml.example

Test plan

• Verify pytest tests/test_config.py passes
• Verify openkb init no longer prompts for PageIndex API key env var
• Verify PageIndex Cloud still works with PAGEINDEX_API_KEY in .env

[rejojer]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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