fix(tui): keep background marker with subagent label

[kitlangton]

Summary

• move the (background) qualifier next to the subagent identity instead of appending it after a potentially long description
• prevent awkward wrapped output such as a dangling ( followed by background) on its own line
• add focused formatting coverage for foreground and background title variants

Rendering

Before:

Explore Task — Audit background task while its child is actively inspecting the
  deeply nested inline renderer implementation (
  background)

After:

Explore Task (background) — Audit background task while its child is actively
  inspecting the deeply nested inline renderer implementation

Stack

• Builds on fix(tui): clarify inline subagent rows #30051, which introduces the inline subagent row presentation this adjusts.
• feat(tui): add internal debug frames #30269 is stacked above this PR and provides the internal debug-frame harness used for the narrow-width visual check.

Verification

• git diff --check
• bun run test -- test/cli/tui/inline-tool-wrap-snapshot.test.tsx test/cli/tui/debug-frame.test.ts on stacked child feat(tui): add internal debug frames #30269 (12 pass, 6 snapshots)
• bun typecheck from packages/opencode on stacked child feat(tui): add internal debug frames #30269
• live cellshot narrow-width render through feat(tui): add internal debug frames #30269 debug frames confirms (background) stays attached to the identity while long descriptions wrap cleanly

Scoped oxlint remains blocked by the existing malformed .oxlintrc.json configuration on the base branch.