rename pi_provider to subprovider in pi-coding-agent target config

[christso]

Problem

The `pi_provider` field in `pi-coding-agent` target config is misleadingly named. It maps directly to Pi CLI's `--provider` flag (e.g. `openrouter`, `anthropic`, `openai`, `github-copilot`).

The name `pi_provider` was a workaround because agentv already uses `provider` for its own provider kind (e.g. `pi-coding-agent`). The code already has multiple aliases (`pi_provider`, `piProvider`, `llm_provider`) which shows the name was never settled.

Pi CLI naming

Pi CLI uses `--provider` for this field:
```
pi --provider openrouter --model openai/gpt-5.4 "..."
pi --provider github-copilot --model ...
pi --provider anthropic --model claude-sonnet-4-20250514 "..."
```

Proposed Change

Rename to `subprovider` — makes clear it's the provider within the pi-coding-agent, not the agentv provider itself.

Before:
```yaml

• name: pi
  provider: pi-coding-agent
  pi_provider: openrouter
  model: openai/gpt-5.4
  ```

After:
```yaml

• name: pi
  provider: pi-coding-agent
  subprovider: openrouter
  model: openai/gpt-5.4
  ```

Scope

• `packages/core/src/evaluation/providers/targets.ts` — update field parsing (keep `pi_provider`/`piProvider`/`llm_provider` as deprecated aliases)
• `examples/features/.agentv/targets.yaml` — update example
• Docs site if referenced

Notes

• Could also apply to `pi-agent-sdk` if it's not removed first (see deprecation notices added in feat(skill-trigger): multi-provider detection + workspace-scoped skills #702)
• The `llm_provider` alias already exists, suggesting this rename was considered before