Add 1Password credential loading (tw auth op)

## Feature
Add `tw auth op` command that loads credentials from 1Password and exports them as env vars.

```bash
eval \$(tw auth op --item 'X.com API (govindani)' --vault 'Local AI Agent')
```

Should output:
```
export TW_CONSUMER_KEY="..."
export TW_CONSUMER_SECRET="..."
export TW_ACCESS_TOKEN="..."
export TW_ACCESS_TOKEN_SECRET="..."
```

Requires `op` CLI to be installed and authenticated.