fix(session): bound diff summary payload

[ponponon]

Issue for this PR

Closes #33106

Type of change

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

What does this PR do?

Caps session summary diffs before generating, persisting, and serving them so a pathological workspace diff cannot bloat session or message JSON.
The snapshot diff path now stops once the summary budget is reached, and oversized rows are trimmed on read and write as a safety net.

How did you verify your code works?

• bun test test/session/summary-diffs.test.ts
• Targeted regression coverage for Snapshot.diffFull(..., { limit })
• Inspected the affected SQLite session data to confirm the oversized payload source
• Repo-wide bun test / pre-push typecheck still hit unrelated pre-existing module-resolution errors

Screenshots / recordings

N/A

Checklist

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

[github-actions]

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

Based on my search, I found several related PRs addressing similar issues with diff handling and session summaries:

Potentially Related PRs:

1. PR fix(ui): guard session review diffs #33664 - fix(ui): guard session review diffs - Guards diffs in session review UI
2. PR fix(session): stop retaining full file-diff patch text in summaries #32410 - fix(session): stop retaining full file-diff patch text in summaries - Addresses the same core issue of reducing diff payload in session summaries
3. PR fix(desktop): prevent large diff timeline hangs #33122 - fix(desktop): prevent large diff timeline hangs - Addresses performance issues with large diffs
4. PR fix(app): avoid O(n^2) dedup hang on large diff summaries #30441 - fix(app): avoid O(n^2) dedup hang on large diff summaries - Related performance optimization for large diff summaries

The most relevant is PR #32410 which appears to have previously addressed the same problem of retaining file-diff patch text in summaries. You may want to check if this PR was merged and whether this new PR (33892) is refining or extending that fix further.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

Automated PR Cleanup

Thank you for contributing to opencode.

Due to the high volume of PRs from users and AI agents, we periodically close older PRs using automated criteria so maintainers can focus review time on the most active and community-supported contributions.

This PR was closed because it matched the following cleanup criteria:

• The PR was created more than 1 month ago
• The PR had fewer than 2 positive reactions
• Positive reactions are counted as thumbs-up, heart, celebration, or rocket reactions on the PR

PRs created within the last month are not affected by this cleanup.

If you believe this PR was closed incorrectly, or if you are still actively working on it, please leave a comment explaining why it should be reopened. A maintainer can review and reopen it if appropriate.

Thanks again for taking the time to contribute.