fix(session): restore summary body generation

[kynnyhsap]

Restores the summary body generation that was accidentally removed in 71a7ad1.

Fixes #11133

What does this PR do?

That commit ("fix model selection in title generation to use user's model instead of assistant's") removed the assistantMsg/small variable and deleted the entire body generation block as collateral damage instead of updating it. The "summary" agent and prompt still exist but are never called.

This means userMsg.summary.body is always undefined since v1.1.15, which breaks downstream consumers that rely on the summary body.

Solution

Re-adds the body generation block after title generation in summarizeMessage(), adapted to the current code:

• Uses userMsg.model.providerID instead of the removed assistantMsg.providerID
• Uses MessageV2.toModelMessages() (updated API with model param)
• Adds if (summaryAgent) null guard consistent with title's if (!agent) return
• Keeps the diffs.length > 0 optimization from 651a10d

How did you verify your code works?

• tsgo --noEmit passes for opencode package
• Full build succeeds (all platforms)

[github-actions]

Thanks for your contribution!

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Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

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Closing this pull request because it has had no updates for more than 60 days. If you plan to continue working on it, feel free to reopen or open a new PR.