fix(opencode): preserve task metadata after truncation

[HyunggyuJang]

Summary

• Truncate task output before appending <task_metadata> so session_id survives truncation.
• Add a unit test that asserts metadata is present when output is truncated.

Testing

• bun test test/tool/task-truncation.test.ts

Issue

Fixes #11903

[github-actions]

Hey! Your PR title Preserve task metadata after truncation doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

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

No duplicate PRs found

[github-actions]

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.