[task] Render GitHub issue list in TUI

[phongndo]

Goal

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

Why this matters

Issue triage is a common contributor and maintainer workflow. A focused issue list makes Nexgit useful before the full dashboard is complete.

Command/API target

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

Depends on

• [task] Add app-server issue list and view methods #8 for app-server issue data.
• [task] Render GitHub workflow queue in TUI #13 for the first TUI workflow queue structure.

Out of scope

• Do not implement issue creation, labels, or assignment actions.
• Do not build desktop UI.

Definition of done

• TUI has an issue list view with number, title, labels or status, and selection state.
• Empty, loading, no-auth, and error states are readable.
• The selected issue 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 PR list in TUI #17 [task] Render GitHub PR 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.