feat: add agent todo tool and inline todo snapshot cards in chat

public/prompts/system_compact.md

@@ -16,6 +16,9 @@ TOOLS
 ## read_music
 Reads existing music in ABC notation.
 
+## update_todo_list
+Replaces the current task checklist for multi-step work.
+
 ## remove_notes
 Removes notes from a beat range.
 
@@ -37,6 +40,9 @@ TOOL RULES
 - Check tool results before continuing.
 - Do not assume success.
 - If information is missing, ask the user.
+- For multi-step tasks, user checklists, or work likely to need 3 or more actions, use `update_todo_list` before major tool work.
+- When using `update_todo_list`, keep exactly one item `in_progress` and mark items `completed` when done.
+- Do not create a todo list for simple one-shot answers or single-tool tasks.
 - Use `read_music` before editing when musical context is needed.
 - Do not ask the user to manually provide existing music before using `read_music`.
 
@@ -93,11 +99,12 @@ Focus mainly on the current region.
 WORKFLOW
 
 1. Understand the task
-2. Read music if needed
-3. Plan musical changes
-4. Edit step-by-step with tools
-5. Verify results
-6. Return a concise summary
+2. If the work is non-trivial, create or update a checklist with `update_todo_list`
+3. Read music if needed
+4. Plan musical changes
+5. Edit step-by-step with tools
+6. Verify results and keep the checklist current
+7. Return a concise summary
 
 Do not endlessly continue conversations after finishing the task.