feat(tools): add dedicated todo tool for agent task tracking (#196)

- Add new todo tool with create, list, update, mark_done, mark_pending, delete actions
- Each subagent has isolated todo storage keyed by agent_id
- Support bulk todo creation via JSON array or bullet list
- Add TUI renderers for all todo actions with status markers
- Update notes tool to remove priority and todo-related functionality
- Add task tracking guidance to StrixAgent system prompt
- Fix instruction file error handling in CLI

strix/agents/StrixAgent/system_prompt.jinja

@@ -111,6 +111,17 @@ OPERATIONAL PRINCIPLES:
 - Try multiple approaches simultaneously - don't wait for one to fail
 - Continuously research payloads, bypasses, and exploitation techniques with the web_search tool; integrate findings into automated sprays and validation
 
+TASK TRACKING:
+- USE THE TODO TOOL EXTENSIVELY - this is critical for staying organized and focused
+- Each subagent has their own INDEPENDENT todo list - your todos are private to you
+- At the START of any task: Create todos to break down your work into clear steps
+- BEFORE starting a task: Mark it as "in_progress" - this shows what you're actively doing
+- AFTER completing a task: Mark it as "done" immediately - don't wait
+- When you discover new tasks: Add them as todos right away
+- ALWAYS follow this workflow: create → in_progress → done
+- A well-maintained todo list prevents going in circles, forgetting tasks, and losing focus
+- If you're unsure what to do next: Check your todo list first
+
 EFFICIENCY TACTICS:
 - Automate with Python scripts for complex workflows and repetitive inputs/tasks
 - Batch similar operations together