[FEATURE] Expose sub-agent model in the subagentStatusLine payload

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Sub-agents can run on a different model than the main session (an agent definition with a model override, a Task-tool spawn on a specific tier, forked sub-agents, etc.). When several sub-agents are running, I want to see which model each one is on directly in the agent panel, but there is no way to surface it: the model is not passed to subagentStatusLine.

The main statusLine payload does not help either — its model object reflects only the top-level session and does not change when a sub-agent is focused. So no custom status line can display a sub-agent's model today.

Current behaviour (verified on Claude Code 2.1.199)

The subagentStatusLine command receives a tasks array where each task object has exactly these fields:

id, type, status, description, label, startTime, tokenCount, tokenSamples, cwd

There is no model field on the task object, and no model information anywhere else in the per-task payload.

Proposed Solution

Add a model object to each entry in the subagentStatusLine tasks array, mirroring the shape already used by the main statusLine payload:

{
  "id": "a265ffabe55c12d85",
  "type": "local_agent",
  "status": "running",
  "description": "...",
  "label": "...",
  "startTime": 1783036392619,
  "tokenCount": 56805,
  "tokenSamples": [56805],
  "cwd": "/path/to/worktree",
  "model": {
    "id": "claude-haiku-4-5-20251001",
    "display_name": "Haiku 4.5"
  }
}

Reusing the existing { id, display_name } shape keeps it consistent with model in the main statusLine payload, so the same rendering logic works in both places.

Alternative Solutions

None are viable today. The model is absent from both the main statusLine payload (per-session only) and the subagentStatusLine tasks array, so there is no data a custom status line could use to derive or display a sub-agent's model.

Priority

Low - Nice to have

Feature Category

Configuration and settings

Use Case Example

1. A coordinator session spawns several sub-agents, some on Haiku for cheap mechanical work and some on Opus for harder reasoning.
2. A custom subagentStatusLine script renders one row per running sub-agent (name, status, token count).
3. With this feature, that row could also show the model, so at a glance I can confirm each sub-agent is on the intended tier.

Additional Context

Related: #65792 requests per-agent context-window usage in the same panel - same area, different field. Adding model alongside would fit naturally into the same per-task payload.