The todo tool only had set/view/update/clear. Adding a step that surfaced
mid-task meant either 'set' (which replaces the plan and resets every status
to pending) or 'replan' (1–3 LLM calls) — both heavy for a single new step, so
the agent usually skipped recording it. Add a cheap 'add' op:

- navi/tools/todo.py: op 'add' (tasks: list[str]) appends new _Task entries to
  the end of the existing plan in pending status; existing steps and their
  statuses are untouched. Requires an existing plan (points to 'set'
  otherwise). Schema enum + op description + tool description updated.
- system_prompt.txt: tell the agent to record new subtasks with 'todo add'
  right away (not held in head, not a replan); clarify the 'small todo edit'
  line — add via 'todo add', drop/merge/reorder via 'set' (re-apply statuses).
- renderers/todo.py: TodoStartedRenderer renders the add card
  ('→ todo · add (N)') listing the new pending steps, not a JSON dump.

Tests: add appends + preserves statuses / requires existing plan / requires
tasks; renderer add card + empty-tasks. Full suite: 1005 passed, 1 skipped.