fix(using-git-worktrees): auto-create .worktrees/ instead of global fallback

[RaviTharuma]

Summary

• Remove ~/.config/superpowers/worktrees/ as global fallback directory
• When no .worktrees/ or worktrees/ exists, auto-create .worktrees/ in the project root
• Ensure the directory is gitignored before proceeding
• Update Quick Reference, Common Mistakes, and Red Flags sections accordingly

Problem

In autonomous modes (autopilot, subagent-driven-development, ralph), agents don't ask the user where to create worktrees — they silently pick the global fallback ~/.config/superpowers/worktrees/<project>/. This causes:

1. 1GB+ of worktrees accumulating in a config directory
2. Code changes, tests, and builds running inside ~/.config/
3. Confusing file layout — code modifications buried in a config folder

Solution

Always keep worktrees local to the project. If .worktrees/ doesn't exist, create it automatically instead of falling back to a global directory. This is safe because:

• .worktrees/ is a common convention (hidden directory, like .git)
• The skill already verifies gitignore status and adds the entry if missing
• Project-local worktrees are easier to discover, manage, and clean up

Fixes #1091