feat(tui): dedicated renderers for background-agent tools (#3103)

[aheritier]

Summary

The four background-agent tools (run / list / view / stop_background_agent) had no entry in the tool factory, so they fell through to defaulttool and rendered as raw JSON / text blobs. This gives them dedicated, presentation-only renderers so a dispatched fleet reads like delegations. Builds on #3102.

• run_background_agent → a delegation-style card root dispatches worker (accent badges) + the task text, led by the same status-aware fixed-width icon as the transfer_task card (TUI: make a single delegation unambiguous (transfer_task) #3101).
• view / stop_background_agent → SimpleRendererWithResult showing the task id (argument) + the formatted result.
• list_background_agents → shows the task listing. (The plan suggested NoArgsRenderer, but that drops the result — the only useful payload until a live surface lands — so the listing is preserved.)

No runtime change. The live "Background agents (N)" sidebar panel (Appendix C.2) is intentionally out of scope here and remains approval-gated.

Screenshots

run_background_agent fans out to several workers in parallel — each renders as a root dispatches worker card:

list_background_agents shows the live fleet (three workers running concurrently):

Tests

• pkg/tui/components/tool/backgroundagent — run per-ToolStatus glyph + fixed-width invariant; view/stop show task id + result; list shows its result; ANSI-stripped.

task build / task lint clean; touched-package tests pass (same unrelated teamloader DMR failure as #3101).

Stack

Top of the stack — stacked on #3102 (← #3101). Tracking #3104.

Refs #3103

[docker-agent]

Assessment: 🟢 APPROVE

This PR adds dedicated TUI renderers for the four background-agent tools (run_background_agent, list_background_agents, view_background_agent, stop_background_agent). The implementation correctly mirrors the transfertask sibling pattern, and factory registration is complete and correct — all four tool-name constants map to the right constructors.

No high or medium severity bugs found. The drafter identified five low-severity observations:

1. renderRun uses json.Unmarshal directly rather than the partial-JSON toolcommon.ExtractField path — streaming partial payloads produce an empty params.Task (card renders an orphaned icon). The transfertask sibling has the same limitation, so this is consistent with the codebase.
2. iconWidth computed from lipgloss.Width of an ANSI-styled string is correct today but would silently misalign wrapped-line indents if margin conventions changed.
3. renderList intentionally surfaces error-status content as a result, which is reasonable but asymmetric with NoArgsRenderer.
4. TestRunCard_StatusIcon hard-codes spinner frame "⠋", creating a fragile coupling to spinner implementation details.
5. TestRunCard_IconWidthIsStable does not assert len(parts)==3 before indexing parts[len(parts)-1], so a degenerate render could silently measure the wrong column.

None of these warrant blocking the PR. Test coverage for status-icon transitions, icon-width stability, view/stop task-ID extraction, and list result visibility is solid.

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

One medium-severity finding in the new backgroundagent renderer package. The PR correctly mirrors the transfertask pattern and the factory registration is complete. The finding is a known streaming limitation that toolcommon.ParseArgs was designed to address.