fix(session): use idle framing when a checkpoint rebuild follows a completed stop

[YOMXXX]

Summary

Fixes #1915 (part 1). After a finite task completes, a checkpoint rebuild unconditionally reinjected "Pick up the last task as if the break never happened" plus resume reminders, so the main agent re-verified already-finished work → token growth → repeated writers/rebuilds → keep-alive loop.

Root cause: packages/opencode/src/session/checkpoint.ts renderRebuildContext() Section 10 always appended the hard resume text, and Section 11 used a resume-oriented stopReminder() for assistant + finish === "stop". No idle/done branch.

Fix: gate the resume framing on the tail:

• Compute done = lastMessageInfo?.role === "assistant" && lastMessageInfo.finish === "stop".
• Section 10: when done, use an idle framing ("if it fulfilled the user's request, do not reopen or re-verify completed work — wait for the user's next input"); otherwise keep the resume text.
• Section 11: assistant/stop → new idleReminder() (do-not-reopen + progress.md guidance so a genuinely mid-task / truncated / cancelled agent still knows where to judge); tool-calls → autonomousLoopReminder() (unchanged); tool → toolResultContinueReminder() (unchanged).
• Removed the now-unused stopReminder.

Test Plan

• New tests: assistant/stop → no "Pick up the last task", contains "wait for the user"; assistant/tool-calls → keeps resume framing.
• Updated checkpoint-rebuild-v3.test.ts (its old assertion referenced the removed stopReminder text).
• bun test test/session/checkpoint-render-verify.test.ts test/session/checkpoint-rebuild-v3.test.ts — 22 pass; bun typecheck clean.

Notes

• Issue's suggested parts 2 (writer idle/done rules) and 3 (LastMessageInfo/rebuildMode) are follow-ups; part 1 alone stops the reopen loop per the issue.