feat(session): add summary-diff opt-out config

[ualtinok]

Issue for this PR

Closes#26035

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds session.summarize config so users can opt out of automatic session diff summarization during prompt processing.

By default nothing changes. When set to false, opencode skips the two automatic SessionSummary.summarize(...) callers used during prompt runs. This avoids broad session-history hydration for diff metadata when responsiveness is more important than automatic per-turn/session diff summaries.

It does not disable prompt execution, title generation, compaction, tool calls, or revert/manual diff behavior.

Example:

{
"session": {
"summarize": false
}
}

How did you verify your code works?

From packages/opencode:

• bun typecheck
• bun test test/config/config.test.ts
• bun test test/session/prompt.test.ts

The push hook also ran root bun turbo typecheck successfully.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[ualtinok]

@Hona Could you recheck now? Thanks

[elmagow]

Can we have this reviewed? thanks

[ualtinok]

#30127 landed, making this PR obsolete. Thanks @thdxr