Add thinkback plugin for Year in Review animation

Adds the thinkback plugin - a personalized Year in Review ASCII animation
generator for Claude Code users.

Features:
- Multiple vibes: cozy, awards show, morning news, RPG quest
- Quick generation with templates or deep dive with personalized narratives
- Comprehensive animation helpers for backgrounds, transitions, particles
- Stats extraction from Claude Code usage history

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/thinkback/.claude-plugin/plugin.json

@@ -0,0 +1,8 @@
+{
+  "name": "thinkback",
+  "description": "Generate a personalized Year in Review ASCII animation celebrating your year with Claude Code",
+  "author": {
+    "name": "Anthropic",
+    "email": "support@anthropic.com"
+  }
+}