[task] Add app-server PR checks method

[phongndo]

Goal

Expose GitHub PR check status through the app-server.

Why this matters

Checks are a core workflow signal for both TUI and desktop dashboards. Contributors should not need to open GitHub web or run separate gh commands to understand whether a PR is blocked by CI.

Command/API target

Add behavior for this app-server method:

github.pr.checks

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 check data types.
• [task] Add app-server PR list and view methods #9 for PR lookup behavior.
• [task] Add GitHub API fixture tests #10 for fixture tests.

Out of scope

• Do not rerun checks or mutate workflow runs.
• Do not implement a full merge-readiness engine.
• Do not build TUI or desktop rendering here.

Definition of done

• App-server can return check names, states, required/optional status where available, and links where available.
• Pending, passing, failing, missing, and unavailable states are not collapsed into one vague value.
• Missing permissions or unavailable check data return clear errors.

Verification

• cargo fmt --all --check
• cargo check --workspace
• Fixture or focused tests cover at least passing, failing, and pending check states.

Contributor notes

This is read-only. Write actions like rerun belong in the Workflow Actions milestone.