feat: implement exitplanmode tool for plan mode workflow

[sandys]

Previously, plan mode prompts referenced an ExitPlanMode tool that didn't exist, causing the AI to get stuck in loops when trying to exit planning.

Changes:

• Add exitplanmode tool that accepts a plan parameter and signals transition out of planning phase
• Register tool in registry, filtered to only be available in plan mode
• Simplify plan mode prompts to clearly explain the workflow
• Add guidance for interpreting future-tense user instructions (e.g., "remember to add tests") as plan items, not immediate actions

This fixes the issue where users saying things like "make sure you write a markdown before starting" would cause the AI to loop indefinitely, unable to reconcile the instruction with plan mode constraints.