feat: Add variety to loading messages with shimmer animation and plugin hook

Summary

Add rotating loading messages with a shimmer animation to replace the static "thinking..." spinner text. Include a plugin hook (register_loading_messages callback) so forks/plugins can inject their own org-specific messages.

What this adds

Core (OSS)

• loading_messages.py — 160+ fun/silly messages across 5 categories (puppy, dev, fun, action, standalone)
• shimmer.py — Travelling highlight animation effect for Rich Text
• Plugin registry (register_messages / unregister_messages) for custom message injection
• register_loading_messages callback phase in callbacks.py
• Spinner rotates through shuffled message deck per cycle (no repeats until deck exhausted)
• Updated status_display.py to use shared message pool

Plugin hook (for forks)

Forks can register a callback to inject org-specific messages:

from code_puppy.callbacks import register_callback
from code_puppy.loading_messages import register_messages

def _add_my_messages():
    register_messages('my_org', ['doing org things...', 'synergizing...'])

register_callback('register_loading_messages', _add_my_messages)

Motivation

Inspired by Claude Code's ~300 creative loading verbs. Makes the spinner more fun and engaging while keeping core and org-specific content cleanly separated via the plugin system.