Add CorpusConfig to the main Config model for unified configuration

[himanshu231204]

Description

Currently, the corpus auditing configuration (\CorpusConfig) exists as a separate model and is not integrated into the main \Config\ model. This means users can't configure corpus checks through a single \config.yaml\ file — they have to use CLI flags.

What to Do

1. Integrate \CorpusConfig\ into the main \Config\ Pydantic model
2. Update the config loader to parse the \corpus:\ section
3. Update the CLI \�udit\ command to accept config from the file
4. Add tests for the new integration

Files to Look At

• \openagent_eval/config/models.py\ — main config models
• \openagent_eval/corpus/base.py\ or \openagent_eval/corpus/models.py\ — existing CorpusConfig
• \openagent_eval/cli/commands/audit.py\ — CLI command

Good First Issue ✅

A more involved task that connects multiple parts of the system.