[task] Compare Nexgit with GitHub Desktop and gh-dash in docs

[phongndo]

Goal

Document Nexgit's product positioning against existing tools without overstating novelty.

Why this matters

GitHub Desktop and gh-dash already exist. Nexgit should be honest that its niche is an open-source Rust TUI plus Electron desktop over the same local GitHub workflow engine.

Command/API target

Update product docs with concise references to GitHub Desktop PR review limitations and gh-dash's terminal dashboard strengths.

Depends on

None. This can be claimed immediately.

Out of scope

• Do not write marketing copy that claims Nexgit is the first GitHub TUI or first GitHub desktop client.
• Do not add implementation code.
• Do not introduce stacked PR positioning.

Definition of done

• Docs explain what GitHub Desktop already does well.
• Docs explain what gh-dash already does well.
• Docs state Nexgit's intended local TUI + desktop workflow niche.

Verification

• pnpm format:prettier:check
• Links should include GitHub Desktop PR docs, GitHub Desktop PR review request #13262, and gh-dash.

[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.