[Bug] Resume/fork includes completed-turn thinking signatures, inflating context usage to near limits

Bug Description
Resume/fork replays all completed-turn thinking signatures and counts them against context. Forked session showed 967k/1M Messages; ~700k was thinking signatures (Opus 4.8, text omitted, signatures remain). Stripping all but the last 3 turns' thinking → 195k, and it resumed fine — so completed-turn thinking is droppable. Please omit completed-turn thinking when assembling the request on resume (keep the active round + keep them in the stored transcript for forking), or apply clear_thinking. As-is, usable context on long resumed sessions is far below 1M.

Environment Info

• Platform: linux
• Terminal: gnome-terminal
• Version: 2.1.170
• Feedback ID: d68b3ef1-e9a6-41f9-b10c-e77a1b7414c9

Errors

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