[FEATURE]: Render task-style transcript rows for child sessions, not only task tool parts

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Task-style transcript rows — the per-child row with nested tool calls, live status, the subagent viewer, and the "view subagents" hint — are gated on the message containing a tool part named task whose state metadata carries the child session id. Only the built-in task tool writes that shape, so child sessions created any other way get no transcript row at all, even though child navigation (ctrl+x down) already reaches them because it keys off session records' parentID, not tool parts.

Verified against current main (3dd1b30):

• packages/opencode/src/tool/task.ts (~183): the task tool writes metadata: { parentSessionId, sessionId, model, background? } via ctx.metadata — the only writer of task-shaped parts.
• packages/tui/src/routes/session/index.tsx: toolDisplay() maps a hardcoded tool-name set including "task" (~2635) to the Task component (~1765); Task reads metadata.sessionId (~2222) and syncs the child's messages to render nested tool calls; the "view subagents" hint row requires a task part in the message (~1509).
• The same route already computes children from session records' parentID (~206) — the data for generic rendering is already in hand.
• Plugins cannot approximate this themselves: TuiHostSlotMap has no transcript/message slot (packages/plugin/src/tui.ts, ~455), the server API has no endpoint to write message parts, and prompt-input agent/subtask parts don't render (PART_MAPPING covers only text/tool/reasoning, ~1578).

Proposal: render a task-style row per child session of the current session — the children list the route already builds — with title from the session row, live state from session_status[childID], and the same enterChild navigation the Task row uses. Anchor the group under the latest assistant message, the same place the existing hint row sits. Keep task-part rows exactly as they are.

Motivation from the real world: ultraopen, a workflow-orchestration plugin, spawns child sessions via client.session.create({ parentID, ... }) and prompts them. Users can navigate into each child, but the transcript shows nothing about them — one workflow tool call, then silence — while a task-tool fan-out in the same UI shows a row per child.

Start in packages/tui/src/routes/session/index.tsx, reading the existing children list, Task component, toolDisplay(), and enterChild navigation; inspect packages/opencode/src/tool/task.ts to compare the current task-part metadata. Render child-session rows under the latest assistant message with the child title and session_status state, while preserving existing task-part rows and navigation.