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/README.md

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+# Thinkback Plugin
+
+Generate a personalized "Year in Review" ASCII animation celebrating your year with Claude Code.
+
+## What It Does
+
+Creates a custom ASCII art animation showcasing your coding statistics and achievements with Claude Code. Features:
+
+- **Multiple vibes**: Cozy fireplace, Awards show, Morning news, RPG quest
+- **Personalized stats**: Commits, conversations, projects, and more
+- **Animation helpers**: Backgrounds, transitions, particles, text effects
+- **Quick or deep generation**: Template-based or fully personalized narratives
+
+## Usage
+
+```
+/thinkback
+```
+
+Claude will guide you through:
+1. Extracting your Claude Code usage statistics
+2. Choosing a vibe for your animation
+3. Selecting which projects to highlight
+4. Generating your personalized Year in Review animation
+
+## Modes
+
+- `mode=generate` (default) - Create a new Year in Review
+- `mode=edit` - Modify an existing animation
+- `mode=fix` - Validate and fix errors in existing animation
+- `mode=regenerate` - Start fresh
+
+## Authors
+
+Thariq Shihipar (thariq@anthropic.com)