GithubRepo has no code access, which blocks using a repo connection for anything but issues/PRs

`packages/gatekeeper-github/src/types.d.ts` already flags this:

```ts
export interface GitHubRepo {
// TODO: Add methods to access code. Maybe represent that as `GitRepository`. For now we only
// expose issues and PRs

Filing it as an issue since it's a blocker rather than a nice-to-have: without file reads, a connected repo isn't nearly as useful as one that can actually see the code. Automating a loop from error -> agent -> bug fix PR seems a natural place for a gadget, but that requires code access.

Worth noting the OAuth grant is already repo (github.ts:279), so file access wouldn't need a scope change or re-consent for existing connections.

Read packages/gatekeeper-github/src/types.d.ts and inspect the existing OAuth grant at github.ts:279. Define the repository code-access interface, then verify that an existing repo connection can read files without a scope change while retaining issue and pull-request access.

Written by the indexing model from the issue text.