fix: edit existing HTML instead of regenerating from scratch

[GeneralJerel]

Summary

• Adds edit-vs-regenerate decision logic to the agent's system prompt so it modifies previous HTML in place when the user requests changes to an existing visualization
• Adds a mode parameter ("new" | "edit") to plan_visualization tool as a forcing function for the edit/new decision
• Adds editing guidance to the master playbook skill for surgical, minimal changes

Closes #77

Test plan

• Generate a visualization (e.g., "Create a bar chart showing quarterly revenue")
• Send a follow-up edit (e.g., "Change the colors to blue and add a title") — verify agent calls plan_visualization with mode="edit" and modifies rather than regenerates
• Send an unrelated request (e.g., "Show me a pie chart of market share") — verify agent generates fresh with mode="new"
• Confirm no regressions on first-time visualization generation

🤖 Generated with Claude Code