Add good/bad examples to rules that lack them

[bbatsov]

Just backfilling a bunch of rules with no examples.

• Add [source,ruby] code examples with # bad and # good annotations to 17 rules that previously had text-only descriptions
• Examples follow the existing patterns used throughout the guide
• Rules that are inherently textual (e.g., "Use UTF-8", "Write comments in English", "SOLID design") were intentionally skipped

Rules updated

1. One-line Cases — when x then ... compact form vs multi-line
2. Semicolon in when — when x; ... vs when x then ...
3. Multi-line Ternary Operator — multi-line ?: vs if/else
4. BEGIN Blocks — BEGIN {} vs top-level code
5. Too Many Params — many positional params vs keyword args
6. Private Global Methods — bare method vs Kernel private method
7. Hash Space — #comment vs # comment
8. No Mutable Keys — mutable string key vs symbol/frozen key
9. No Modifying Collections — mutation during iteration vs select/reject
10. map/find/select/reduce/include?/size — Smalltalk aliases vs preferred names
11. Time.now — Time.new vs Time.now
12. %s — %s(symbol) vs :"symbol"
13. No Needless Metaprogramming — unnecessary define_method vs plain def/attr_reader
14. No Monkey Patching — reopening core class vs refinements
15. No Param Mutations — mutating param vs returning new object
16. Three is the Number Thou Shalt Count — deeply nested blocks vs extracted methods
17. No Optional Hash Params — hash param vs keyword args

[bbatsov]

I've generated the examples automatically from the rules and the results are pretty good IMO.