Plan mode improvements: Opencode, citation, auto-settle

[casassg]

Hi folks, would like to start a thread around some small changes for plan mode. I see plan mode is marked as legacy, so let me know if that means it's unlikely to receive improvements or wether improvements are open for ideas:

1. OpenCode plan recognition. An OpenCode plan-mode turn ends as a plain message, so there is no plan card and no Implement follow-up. In plan mode only, append the same <proposed_plan> formatting rules Codex gets to the OpenCode system prompt, and have the adapter split the block out of the streamed text. The message keeps only the prose, and the block becomes a normal proposed_plan node, artifact and turn item. A later block supersedes the earlier one.

2. Cite text from proposed plans (web). Only assistant messages can be quoted today. Plan cards become citation sources using the proposed_plan turn item id, so no contract or server change is needed (the citation id is never resolved server-side). Applies to plans from every provider. This allows users to give specific feedback to the agent about plan items.

3. Settle the source thread after implementing in a new thread. The planning thread is done but stays in the active sidebar. Settle it (using the existing settle action and its Undo notice) once the new thread has started.

Drafts on my fork, with passing focused tests but no screenshots yet: casassg#2 (1), casassg#3 (2), casassg#5 (4).

Happy to drop or reshape any of them.