[Refactor] Refactor Codebase investigator into a toml based agent

[abhipatel12]

As a step towards configuring agents using config files, the codebase investigator should be refactored into a toml that is loaded by default.

Completion Criteria:

• [ ] Codebase investigator is completely defined via a toml file

Bonus: Consider making this a more generica exploration/context gathering subagent