[task] Render GitHub PR list in TUI

[phongndo]

Goal

Add a dedicated TUI pull request list view backed by shared GitHub workflow data.

Why this matters

Maintainers repeatedly scan PR queues for review, CI, and merge status. The TUI should make that queue fast to inspect from the terminal.

Command/API target

Render github.pr.list data in a keyboard-navigable TUI view.

Depends on

• [task] Add app-server PR list and view methods #9 for app-server PR data.
• [task] Render GitHub workflow queue in TUI #13 for the first TUI workflow queue structure.

Out of scope

• Do not implement checkout, merge, or review actions.
• Do not build desktop UI.

Definition of done

• TUI has a PR list view with number, title, branch/base, author, and status where available.
• Empty, loading, no-auth, and error states are readable.
• The selected PR can be identified for later detail-view work.

Verification

• cargo fmt --all --check
• cargo check --workspace
• Run cargo run -p nexgit-cli -- tui and include a screenshot or concise manual observation.

[github-actions]

This issue needs a small update before triage:

• Area must be one of Automation, CLI, Core, Desktop, Design, Docs, Protocol, Tests, or TUI so triage automation can route the issue.

Please edit the issue instead of opening a new one.

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Potentially related existing work:

• [task] Render GitHub issue list in TUI #16 [task] Render GitHub issue list in TUI (open)
• [task] Render GitHub workflow dashboard in desktop #14 [task] Render GitHub workflow dashboard in desktop (open)
• [task] Render TUI PR detail and checks view #18 [task] Render TUI PR detail and checks view (open)

If one of these is the canonical thread, comment there and a maintainer can close this issue with /duplicate #123; otherwise a maintainer can clear the review with /not-duplicate.