tui: show the changed step's text in the todo update call card ...

The todo update call card (→ todo · #3 → done) now shows the text of the
step being changed, plus the validation when present. The LLM's update args
carry only the index, not the task text, so the text is read from the
current plan row before the tool runs and attached to ToolStarted.metadata
(client-rendering only, backward-compatible).

Backend:
- events.ToolStarted gains a metadata dict (mirrors ToolEvent) → to_wire.
- navi/tools/todo: step_text_for_update(index, ctx) resolves the step text
  via _sid/_uid (so sub-agent isolation holds), started_metadata_for_call
  wraps it for both emit sites.
- agent.py (parent) and subagent_runner.py (sub-agent) enrich ToolStarted
  via the shared helper before emitting.

Renderer:
- TodoStartedRenderer update card reads msg.metadata.step_text → shows the
  step text + validation; falls back to 'no validation' on history replay.
- Removed the step-text line from the result card (it now lives in the call
  card) — result card is back to the compact 'plan · X/N done' summary.

Co-Authored-By: Claude <noreply@anthropic.com>