Stop inserting completed-plan cards into conversation history. Plan
progress stays in the existing plan panel instead of gaining a second
transcript presentation when a turn finishes.

- Keep the completed current plan visible in the plan panel, including
its existing **All Done** state.
- Let later plan updates revise that state in place, preserving Markdown
entity identity.
- Preserve **Clear Plan** and the existing next-turn cleanup of
completed rows.
- Remove the completed-plan transcript variant, snapshot calls, card
renderer, and associated search/export/view-state handling.

This was an experimental UI change when we had an update_plan tool in
the Zed agent. But now that this is ACP only, I think this
simplification is warranted, also because there are upcoming changes to
plans in RFD for v1 and v2 that will be id-based and allow future
updates.

Release Notes:

- N/A