feat: Show helpful tips while code-puppy is thinking

Summary

When code-puppy is processing/thinking, display rotating helpful tips to educate users on commands, features, and general AI coding agent best practices. This is especially valuable for users who are new to AI coding harnesses and agent-based workflows, while also serving as a useful reminder system for more experienced users.

Assigned to @nhicks00 and @Nate0-1999 (Nathan Oswalt) — @Nate0-1999 is leading implementation on this one.

Motivation

Right now, the thinking phase is a dead zone — the spinner spins, but nothing useful is communicated. This is a perfect opportunity to passively onboard users and surface features they might not know exist.

Proposed Behavior

• While the spinner is active (i.e. the agent is thinking/processing), rotate through a curated set of tips
• Tips should cover:
  • Slash commands (e.g. /cd, /model, /skills, /agent)
  • Useful workflows (e.g. how to resume sessions, how to use sub-agents)
  • General AI coding agent guidance (e.g. how to write good prompts, when to break tasks down)
  • Code Puppy-specific features (e.g. skills marketplace, plugin system, yolo mode)
• Tips should be distinct from the fun loading verb messages (see #202 / PR #203) — they are informational, not decorative
• Should integrate with the register_loading_messages plugin hook so forks (e.g. Walmart) can inject org-specific tips

Implementation Notes

• Tips list should live in a dedicated module (e.g. tips.py) — separate from loading_messages.py
• Plug into the existing spinner infrastructure (spinner_base.py, console_spinner.py)
• Consider displaying tips differently from the fun verb messages — e.g. prefixed with 💡 Tip: for clear visual distinction
• Should follow the same shuffled-deck pattern (no repeats until all tips shown)
• Plugin hook (e.g. register_tips) so forks can add their own org-specific tips

Acceptance Criteria

• Tips module with a solid set of helpful tips covering commands, features, and AI agent guidance
• Tips display during the thinking/spinner phase, visually distinct from loading verb messages
• Plugin hook for injecting custom tips from forks
• Tests for tip registration, retrieval, and display behavior
• ruff format + ruff check clean
• 95%+ test coverage maintained