Document think-tank directory purpose and usage

## Context
After #1003 and #1004, we have warnings about not putting think-tank content in worktrees, but no clear documentation about what the think-tank directory is for.

## Action Needed
Create a short procedure document at `knowledge/procedures/think-tank.md` that explains:
- Purpose: Personal content that doesn't belong in code (notes, resumes, drafts)
- Location: Always use `/think-tank/` in main repository
- Persistence: Content survives worktree cleanup
- Examples: What belongs there vs what doesn't

Keep it concise - just enough to guide proper usage.

Create knowledge/procedures/think-tank.md. Start by reviewing the context in issues #1003 and #1004, then document the directory's purpose, location, persistence, and examples of appropriate and inappropriate content. Done means the concise procedure clearly guides where personal non-code content belongs.

Written by the indexing model from the issue text.