Improve code quality: LICENSE, CI, deps, exception handling

[he-yufeng]

Summary

• Add MIT LICENSE file (2024, universeplayer) -- the repo previously referenced MIT in the README but had no actual license file
• Add GitHub Actions CI workflow (.github/workflows/ci.yml) -- runs ruff linting on push/PR against Python 3.9 and 3.11, catching syntax errors and undefined names
• Fix missing dependency -- add openai>=1.0.0 to requirements.txt (used by AI_interviewer.py via from openai import OpenAI)
• Narrow 4 overly broad exception handlers in llm_client.py:
  • except Exception: on import -> except ImportError:
  • except Exception as e: on JSON config parsing -> except (json.JSONDecodeError, ValueError, OSError)
  • except Exception as e: on API key file loading -> except (FileNotFoundError, ValueError, OSError)
  • except Exception as e: on HTTP retry loop -> except (requests.RequestException, ValueError, KeyError)
• Update README badges -- replace static badges with clickable ones (Python version, License, GitHub Stars, CI status)

Test plan

• Verify CI workflow passes on GitHub Actions
• Verify pip install -r requirements.txt installs all needed packages including openai
• Verify the LICENSE file renders correctly on the repo page
• Verify README badges display correctly
• Smoke test: python -c "from llm_client import LLMClient" still works (no import errors)

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