EnterPlanMode Tool Enhancement

What: The EnterPlanMode tool description has been significantly expanded with better guidance on when to use it. Details:

• Now explicitly recommends using plan mode proactively for non-trivial tasks
• Adds "New Feature Implementation" and "Code Modifications" categories
• Provides more nuanced examples including "Add a delete button to the user profile"
• Changes from "err on the side of starting implementation" to "err on the side of planning"
• Evidence: nc2 at line 435451