Plan mode: track step completion and surface progress in the plan UI

Problem

Once a plan is approved and execution begins, the harness doesn't track which plan items have been completed. Symptoms:

• The "Plan" button/indicator stays visible after every item in the plan is done — there's no signal that execution has finished.
• The structured plan content (steps, sub-bullets) is only rendered at approval time. During execution it's gone, and the user falls back to reading tool calls / transcript to judge progress.

Suggestion

The plan-rendering UI from the approval step already has the right shape for this — a structured list of steps with hierarchy. Reuse it during execution:

• Track per-step completion as the model works through the plan (the model already signals intent via TodoWrite-style structures; plan items could be tied to todos, or inferred from the model marking them).
• Render the plan inline with completion state (checkmarks / strikethrough / progress bar) while execution is ongoing.
• When all items are complete, dismiss the plan indicator automatically instead of leaving it stuck on.

This turns the plan from a one-shot approval artifact into a live progress view, and fixes the "indicator never clears" bug as a side effect.

Environment

Claude Code, current version (noticed after recent UI refresh — prior plan-rendering UI was preserved but no execution-tracking layer was added on top of it).