fix(backend/copilot): default CHAT_USE_OPENROUTER back to true for OR observability (#13066)

## Why

[#13034](https://github.com/Significant-Gravitas/AutoGPT/pull/13034)
flipped `CHAT_USE_OPENROUTER` to default `false`, routing the copilot
main client direct to `api.anthropic.com`. We need to revert that
default so **everything copilot-related stays on OpenRouter** — main
turn, title generation, simulation, future builder helpers — to keep
unified observability (request/response broadcasting, cost tracking,
per-call telemetry) in one place.

Direct-Anthropic mode remains supported and opt-in for deployments that
explicitly want to bypass the proxy.

## What

- `ChatConfig.use_openrouter` default flipped from `False` back to
`True`.
- Field description updated to reflect that OR is the default and
direct-Anthropic is opt-in.

No other code changes — all the direct-Anthropic plumbing from #13034
(rate card, model normalizer, boot validators, aux client split, cost
recovery) stays intact and is exercised whenever an operator opts into
`CHAT_USE_OPENROUTER=false`.

## How

```bash
# OpenRouter (new/restored default — for observability)
# No env vars required; existing OPEN_ROUTER_API_KEY / CHAT_API_KEY are picked up.

# Direct Anthropic (opt-in — bypass OR proxy)
CHAT_USE_OPENROUTER=false
ANTHROPIC_API_KEY=sk-ant-...
# (optional) CHAT_AUX_API_KEY=sk-or-v1-...   # keep aux on OR
```

### Test plan

- [x] `pytest
backend/copilot/{config,anthropic_rate_card,model_normalize}_test.py` —
77 passed locally
- [x] Field default verified at `backend/copilot/config.py:417` →
`default=True`