[task] Add app-server review comments and threads method

[phongndo]

Goal

Expose GitHub PR review comments or review threads through the app-server.

Why this matters

Review context is one of the places where GitHub Desktop and simple gh commands still push users back to GitHub web or raw GraphQL. Nexgit should make review state visible in local TUI and desktop surfaces.

Command/API target

Add behavior for this app-server method:

github.pr.reviewThreads

If GitHub API limitations force an intermediate shape, document it clearly in the PR.

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 review 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 resolve threads, reply, or submit reviews.
• Do not build inline diff commenting yet.
• Do not build TUI or desktop rendering here.

Definition of done

• App-server can return review thread/comment summaries with file path, line or position where available, author, body excerpt, resolved/unresolved state where available, and URL.
• Missing permissions and unsupported API cases produce clear errors.
• The method is ready for TUI and desktop read-only views.

Verification

• cargo fmt --all --check
• cargo check --workspace
• Fixture or focused tests cover resolved and unresolved review-thread-like data.

Contributor notes

This likely needs GraphQL through gh api graphql. Keep the first version narrow.