feat(retro): sk retro --by-wing/--by-tag — grouped domain health view

[magicpro97]

User Story

As a developer, I want sk retro --by-wing to show freshness/score broken down per domain.

Acceptance Criteria

• --by-wing, --by-tag, --by-room flags in retro.py
• One sub-block per group with freshness bar + entry count
• --json output gains a groups key
• Tests in test_fixes.py (15+)

Priority: MED