[task] Add app-server issue list and view methods

[phongndo]

Goal

Expose GitHub issue list and issue detail data through the app-server.

Why this matters

The TUI and desktop issue queues should be backed by one Rust-owned data path instead of separate frontend GitHub calls.

Command/API target

Add behavior for these app-server methods:

github.issue.list
github.issue.view

Depends on

• [task] Add GitHub API adapter using gh api #4 for the GitHub API adapter.
• [task] Define shared GitHub workflow data types #6 for shared issue data types.
• [task] Add app-server auth and repository status methods #7 for auth and repository context.

Out of scope

• Do not implement issue creation, comments, labels, or assignment mutations.
• Do not build TUI or desktop issue panels here.
• Do not support non-GitHub providers.

Definition of done

• Issue list returns open issues for the current GitHub repo.
• Issue view returns title, body, labels, assignees, milestone, state, and URL where available.
• No-auth, no-repo, and empty-list states are distinguishable.

Verification

• cargo fmt --all --check
• cargo check --workspace
• Manual stdio app-server requests for list and view included in the PR description.

Contributor notes

Use conservative pagination for the first version. Broader filtering can be separate issues.