docs(procedures): add think-tank directory documentation

[atxtechbro]

Problem & Solution

Problem: After implementing warnings about not putting think-tank content in worktrees (#1003, #1004), there was no clear documentation about what the think-tank directory is actually for.
Solution: Created a concise procedure document explaining the think-tank directory's purpose, location, and usage guidelines.
Keywords: think-tank, personal content, worktree, persistence, documentation

Related Issues

Closes #1005

Technical Details

Files changed:

• Added knowledge/procedures/think-tank.md

Key modifications:

• Clear purpose statement for personal content storage
• Explicit location guidance (main repo only, never worktrees)
• Examples of what belongs vs what doesn't
• Concise format for quick reference

Alternative approaches considered:

• Could have added to existing docs, but standalone procedure is clearer
• Kept it intentionally brief per issue requirements

Testing

• Review document for clarity and completeness
• Ensure it addresses the core confusion about think-tank usage

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added/updated tests that prove my fix is effective or that my feature works (if applicable)
• I have updated the documentation accordingly (if applicable)

[amazon-q-developer]

⏳ I'm reviewing this pull request for security vulnerabilities and code quality issues. I'll provide an update when I'm done

[amazon-q-developer]

✅ I finished the code review, and didn't find any security or code quality issues.