[task] Add GitHub API adapter using gh api

[phongndo]

Goal

Add the first Rust adapter for reading GitHub data through gh api.

Why this matters

Nexgit's MVP is GitHub-first, but it should not make contributors solve token storage or OAuth before the app-server data shape is proven. gh api lets Nexgit reuse existing gh auth while keeping GitHub calls inside Rust-owned code.

Command/API target

Create a small adapter that can run REST and GraphQL gh api calls with explicit arguments and return stdout, stderr, and exit status in a testable shape.

Depends on

None. This is the foundation for the GitHub App-Server Data milestone.

Out of scope

• Do not add octocrab or another GitHub client yet.
• Do not implement issue, PR, checks, or review methods here.
• Do not expose GitHub calls directly from desktop renderer code.

Definition of done

• Rust code has one focused GitHub API adapter boundary.
• Successful and failed gh api calls can be represented without panics.
• Later issues can call the adapter instead of spawning gh directly.

Verification

• cargo fmt --all --check
• cargo check --workspace
• A focused test or test double covers success and failure handling.

Contributor notes

This is maintainer-owned because it sets the implementation pattern for later contributor issues.