Subagents should know what model they're running

### Describe the feature or problem you'd like to solve

The main agent's system prompt includes `` so it can self-identify. Subagents launched via the Task tool with a `model` parameter don't receive this — making it impossible to validate model routing in experiments.

### Proposed solution

Propagate a `` block into the subagent's system prompt when spawned with a `model` parameter.

### Example prompts or workflows

1. `"For each available model, create a subagent specifying the model. Ask each to report their model."` — 0/18 could self-identify.
2. `"Run this task on claude-haiku-4.5 and claude-opus-4.6 in parallel. Label which model produced each result."` — impossible without model identity.
3. `"Launch a subagent on gpt-5.4 and confirm it's not running Claude."` — subagent can only say "ChatGPT" generically.

### Additional context

Tested on v1.0.12. Launched 18 explore subagents (all available models), each asked to self-identify:

| Requested Model | Self-Report | Match? |
|---|---|---|
| `claude-sonnet-4.6` | "Claude, likely Sonnet 4" | Close |
| `claude-sonnet-4.5` | "Claude" (no version) | Family only |
| `claude-sonnet-4` | "Claude" (no version) | Family only |
| `claude-haiku-4.5` | "Claude 3.5 Sonnet" | Wrong variant |
| `claude-opus-4.6` | "Claude 4 / Sonnet" | Wrong (Opus != Sonnet) |
| `claude-opus-4.6-1m` | "Claude 4 / Sonnet" | Wrong |
| `claude-opus-4.5` | "Claude" (no version) | Family only |
| `gpt-5.4` | "ChatGPT, OpenAI" | Family only |
| `gpt-5.3-codex` | "OpenAI assistant" | Family only |
| `gpt-5.2-codex` | Refused to answer | Refused |
| `gpt-5.2` | "Don't have enough info" | Unable |
| `gpt-5.1-codex-max` | "ChatGPT" | Family only |
| `gpt-5.1-codex` | "ChatGPT" | Family only |
| `gpt-5.1` | "OpenAI ChatGPT-style" | Family only |
| `gpt-5.4-mini` | "ChatGPT" | Family only |
| `gpt-5.1-codex-mini` | Refused | Refused |
| `gpt-5-mini` | "AI language model" | Vague |
| `gpt-4.1` | "GPT-4-based" | Close |

**0/18 exact matches.** Root cause: model identity is used for routing but never communicated to the subagent.