fix(core): preserve shell output tail

[opencode-agent]

Summary

• add keep: "head" | "tail" to shell output pagination, defaulting to head
• have ShellTool keep the final capture window without reaching into Shell backing storage
• add a regression test proving the tail marker remains, the head marker is omitted, and the saved-output notice remains
• regenerate current client surfaces for the query option

Tests

• bun test test/tool-shell.test.ts
• bun typecheck (core, schema, server, client)

[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.