fix(core): surface background subagent completions in TUI

[CountClaw]

Issue for this PR

Closes #35063

Type of change

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

What does this PR do?

Background subagent completions published synthetic parent-session messages with text only. The V2 TUI renders synthetic rows only when description is non-empty, so completion and failure notices were hidden.

This sets a concise description in SubagentTool.injectCompletion while preserving the structured XML text used for model context. The existing regression test now checks the description.

How did you verify your code works?

Extended packages/core/test/tool-subagent.test.ts to assert the synthetic description. A local Windows run of bun test --timeout 60000 test/tool-subagent.test.ts reached pre-existing temporary-directory EBUSY flakes; the changed path is covered by the updated assertion.

Screenshots / recordings

Not applicable. This affects the background completion and failure path rather than a stable interactive screen.

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[CountClaw]

The PR description now follows the required template and still references #35063. The compliance check passes after the edit.

The needs:issue label remains because this PR targets the non-default v2 branch, so GitHub does not populate closingIssuesReferences for the linked issue. Could a maintainer remove the stale label?

[Kyou12138]

Upstream v2 already sets a non-empty description on background subagent synthetic messages in packages/core/src/tool/plugin/subagent.ts (injectCompletion passes description into runtime.session.synthetic). Issue #35063 is closed (2026-07-29), and the current tests assert the description field.

This PR branch has also drifted badly from v2 (large unrelated history / conflicts). Closing as superseded by the upstream fix. Reopen if a remaining TUI gap is found on current v2.

[Kyou12138]

Closing: fixed upstream on v2; issue #35063 closed.

[CountClaw]

Closing as superseded: #35063 is closed and upstream v2 already passes description into session.synthetic for background subagent completions. Branch also has large unrelated conflicts with current v2.