feat(code-review): TODO tool

[Pertempto]

Add a tool that the LLM can call to find TODO comments with a given filter term.

Behind the scenes, this tool should use something like git grep or similar. We don't want to grep gitignored files.

Update the system prompt to tell the bot to use this tool to look for any TODO comments related to the current issue number, and also any other related terms.

We should have a specific todo array in the bot's output, each todo should be shaped like this:

{
    "file_path": "path/here",
    "line_number": 34,
    "reason": "References issue number #123"
}