✨ feat: Add optional description field to rules

[heumsi]

Summary

• Add an optional description field to Rule that provides human-readable context for why a rule exists
• Show description in violation output when present, helping users understand violations at a glance
• Update violation output format so the arrow line (source → imported) is always on its own line for consistency

Output format

With description:

contexts/boards/domain/models.py:6
    [domain-isolation] Domain layer must remain pure
    contexts.boards.domain → contexts.boards.application.service (local)

Without description (backward compatible):

contexts/boards/domain/models.py:6
    [domain-isolation]
    contexts.boards.domain → contexts.boards.application.service (local)

Config example

rules:
  - name: domain-isolation
    modules: contexts.*.domain
    description: Domain layer must remain pure  # optional
    allow:
      standard_library: [dataclasses, typing]

Changed files

• config.py — Rule dataclass + parsing
• checker.py — Violation dataclass + propagation
• matcher.py — merge_rules preserves description
• reporter.py — output format
• tests/ — test_config, test_checker, test_matcher, test_reporter
• docs/ — config-schema, rules guide, getting-started

Test plan

• description parsed from YAML config
• description parsed from TOML config
• description is None when omitted
• Violation carries rule_description for both allow and deny violations
• merge_rules preserves first rule's description
• Reporter shows description when present
• Arrow line always on separate line
• All 100 tests pass

🤖 Generated with Claude Code