✨ feat: Add optional description field to rules

[heumsi]

Summary

• Add optional description field to the Rule dataclass for human-readable rule explanations
• Update violation output format to always use a two-line structure: [rule_name] (with optional description) on the first line, name (message) on the second line
• Update all docs output examples to match the new format

Output format

# With description
src/domain/service.py:4
    [bool-method-prefix] Bool-returning functions must use a semantic prefix
    validate (expected prefix: is_ | has_ | should_)

# Without description
src/domain/service.py:4
    [bool-method-prefix]
    validate (expected prefix: is_ | has_ | should_)

Test plan

• All 92 existing tests pass
• New tests for reporter with/without description
• New test for config parsing with description field

🤖 Generated with Claude Code