feat: add /btw background session command

[Svtter]

Issue for this PR

Closes #16992
Split from #18635

Type of change

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

What does this PR do?

This PR contains the /btw command work split out from #18635.

It includes:

• /btw ask, /btw todo, and /btw status
• background child session creation and parent-session result reporting
• /btw-specific synthetic message filtering so status/result notes do not leak back into later model turns
• follow-up cleanup by archiving completed or failed background child sessions
• the review-driven refactor that breaks the /btw flow into smaller helpers

How did you verify your code works?

• cd packages/opencode && bun typecheck
• cd packages/opencode && bun test test/session/btw.test.ts test/session/message-v2.test.ts

Screenshots / recordings

N/A - backend/CLI command, no UI changes.

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:

Potential Duplicates Found:

1. PR feat: add /btw background session command #18635 - feat: add /btw background session command #18635

   • Same title: "feat: add /btw background session command"
   • This is the original PR that the current PR (feat: add /btw background session command #18917) is split from (as noted in the PR description)

2. PR feat: add /btw background session command #17198 - feat: add /btw background session command #17198

   • Same title: "feat: add /btw background session command"
   • Appears to be an earlier iteration of this feature

3. PR feat: Add the /btw command #17189 - feat: Add the /btw command #17189

   • "feat: Add the /btw command"
   • Related earlier work on the /btw command

Summary: PR #18917 is explicitly split from PR #18635 (as mentioned in the description), so #18635 is the original parent PR. PR #17198 appears to be an earlier attempt at the same feature. These should be reviewed to ensure no duplicate work is being done across all three PRs.

[Svtter]

@atharvau This is the split-out /btw PR from #18635.

I separated the tool-limit work into #18916, so this PR is just the /btw command flow, background child session handling, and the /btw-specific synthetic message filtering needed for it.

I also re-ran the local verification here and it passes:

• cd packages/opencode && bun typecheck
• cd packages/opencode && bun test test/session/btw.test.ts test/session/message-v2.test.ts

If this scope looks good now, do you think this is ready to merge?

[Svtter]

Closing this PR after self-review: this feature (background session command) falls under core product functionality and did not go through a design review with the core team as required by the contributing guidelines.