feat(plugin): add achievement and badge system for coding habits (#1008)

Add gamification system that tracks coding habit milestones:
- TDD Champion (100 TDD cycles), Agent Master (10+ agents),
  Quality Guard (EVAL 90+), Speed Coder (5 commits/hr), Streak (5 days)
- Achievement progress tracked in ~/.codingbuddy/achievements/
- Celebration message with special buddy face on unlock (stop hook)
- Badge display area in session-start TUI rendering
- 46 tests covering all tracker logic, rendering, and edge cases

Author: JeremyDev87