feat: emit error annotations in GitHub Actions

[vmaerten]

Summary

When a task fails in GitHub Actions, finding the error often means scrolling through hundreds of lines of logs. This PR changes that.

Task now automatically emits ::error:: annotations when running in GitHub Actions (GITHUB_ACTIONS=true). These annotations:

• Appear directly in the workflow summary - no more hunting through logs
• Show the task name - immediately know which task failed
• Link to the error - click to jump to the relevant log line

Before

Errors are buried in logs, easy to miss when you have dozens of tasks running.

After

::error title=Task 'build' failed::exit status 1

The error appears as a red annotation at the top of the workflow run, visible at a glance.

Why this matters

• Faster debugging - See what failed without reading logs
• Better DX in CI - Task becomes a first-class citizen in GitHub Actions
• Zero configuration - Works automatically, no setup required

Test plan

• GITHUB_ACTIONS=true task failing-task emits ::error title=Task 'X' failed::...
• Without GITHUB_ACTIONS=true, no annotation is emitted
• Annotation includes task name when available (TaskRunError)
• Generic errors also emit annotations

[andreynering]

I was just thinking if we want this only for output: group or something, but it will problem work well as is.
You may want to test with different group settings is easy enough.