Combine and simplify query chapters

Merge "Queries in Context" and "Five Query Operators" into a single
streamlined chapter. The new chapter focuses on practical pedagogy:

- Core principles (entity integrity, algebraic closure)
- Quick reference for all five operators with examples
- Comparison with SQL
- Building complex queries step by step

Removes the heavy theoretical/historical content in favor of clear,
actionable guidance for learning the query operators.

Author: claude