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.

Resolution

Renamed to subprovider — makes clear it's the provider within the pi-coding-agent, not the agentv provider itself. All deprecated aliases (pi_provider, piProvider, llm_provider) have been removed.

Before:

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

After:

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

Scope

• packages/core/src/evaluation/providers/targets.ts — renamed field, dropped aliases
• examples/features/.agentv/targets.yaml — updated example
• .agentv/targets.yaml — updated config