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

[phongndo]

Goal

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

Why this matters

PR workflow is the core surface for the future TUI and desktop dashboards. The app-server should own PR lookup so both frontends stay aligned.

Command/API target

Add behavior for these app-server methods:

github.pr.list
github.pr.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 PR data types.
• [task] Add app-server auth and repository status methods #7 for auth and repository context.

Out of scope

• Do not implement PR creation, merge, checkout, or review actions.
• Do not implement checks or review threads here.
• Do not build TUI or desktop PR panels here.

Definition of done

• PR list returns open PRs for the current GitHub repo.
• PR view returns title, body, state, author, branch/base, labels, review decision if available, and URL.
• No-auth, no-repo, missing-PR, 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

Keep the first version read-only and GitHub-only.