feat(opencode): Support background subagents

[tomjw64]

Issue for this PR

Closes #5887

Type of change

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

What does this PR do?

The goal of this PR is to support fire-and-forget/async/background tasks in my best approximation of how Claude Code handles it.

This is done by calling SessionPrompt.prompt async and emitting a new BackgroundTaskCompleted event when the promise resolves. The new SessionBackground component listens for these events (and the idle status event) and takes care of flushing the queue of finished task information to the parent session and forcing it to take it a turn with the results of the completed task(s).

This attempts to be a bit more minimal than #7206 to increase the chances of acceptance. I am a first-time contributor to opencode and I almost certainly overlooked something here. I'm happy to receive any suggestions or review.

How did you verify your code works?

I opened opencode and entered the following prompt:

Run 4 general background subagents in parallel. Each one should use bash to
sleep for an amount of time and then print out a random number. Have them sleep
for 10, 18, 18.2, and 18.2 seconds, respectively. Then print out a chart to be
updated as each task notifies you of its completion.

And verified that the agent (tested with Kimi K2.5 Free):

• Could handle additional user interaction before the first subagent completed.
• Woke after the first subagent completed to update the chart.
• Woke a second time after the second subagent completed to update the chart.
• Woke a third and final time after the third and fourth subagents completed (to test that we buffer the results of two completions but still only give the primary agent a single turn once idled again)

Checklist

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

If you do not follow this template your PR will be automatically rejected.

[github-actions]

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

Based on the search results, I found a related PR that may be relevant:

Potential Related PR:

• PR feat(tui): fire-and-forget async subagent tasks #7206: feat(tui): fire-and-forget async subagent tasks
  • feat(tui): fire-and-forget async subagent tasks #7206
  • This appears to address the same concept of fire-and-forget async subagent tasks, which is the core feature being implemented in PR feat(opencode): Support background subagents #13261.

This PR could potentially be a duplicate or overlapping work on the same feature. You may want to review PR #7206 to understand if it was a previous attempt, partially merged, or if #13261 is a reimplementation with different approaches.

[tomjw64]

Received some review expressing some concern over task cancellation:

1. subagents prompts not cancelled on session delete
2. no way of individually cancelling a session in the TUI

We can address (1) in this PR (update: done), but I will open a separate issue for individual issue cancellation (update: done), so that background tasks can be cancelled without e.g. closing out the entire app.

[allarac]

By the love of god, can someone finally just give us parallel subtasks please? Who's supposed to use opencode effectively in its current state?