Fix ruff lint failures across pkg/ and tests/

[Copilot]

CI was failing on ruff check --select=E,W,F,I and ruff format --check due to accumulated lint issues in source and test files.

Changes

• Import hygiene – sorted import blocks (I001), removed unused imports sys and MagicMock (F401)
• Whitespace – stripped trailing whitespace from blank lines (W293)
• Line length – broke lines exceeding 88 chars (E501) in cli.py, graph.py, metadata.py, and test files; examples:

  # Before
  title = work.get("title", [""])[0] if isinstance(work.get("title"), list) else work.get("title", "Unknown")
  
  # After
  title = (
      work.get("title", [""])[0]
      if isinstance(work.get("title"), list)
      else work.get("title", "Unknown")
  )

• Dead code – removed unused cv_section_start assignment and a bare result.rfind(...) call whose return value was discarded in test_markdown.py
• Formatting – applied ruff format to all 15 files

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[art-test-stack]

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