Add PR review helper commands under gh pr for inline comments and review flows (AI agent use cases)

Summary
The gh CLI is increasingly used by AI agents and automation pipelines to collaborate on pull requests. These workflows benefit from first-class access to:

• reading inline review comments,
• replying to review thread comments,
• opening a pending review, adding inline comments, and submitting the review.

This issue proposes adding focused helper commands under gh pr to cover these workflows cleanly:

Proposed commands

• gh pr see-comments — list inline review comments by review id or latest
• gh pr reply-comment — reply to a review thread comment by comment id
• gh pr review pending open — open a pending review
• gh pr review pending add — add inline review comment(s)
• gh pr review pending submit — submit the pending review

Rationale

• AI agents and bots increasingly participate in code review. Having stable, scriptable surfaces for listing, replying, and managing pending reviews reduces bespoke API calls and aligns with CLI conventions.
• These helpers complement existing gh pr review flows without interfering with branch-based targeting (helpers are nested under review pending …).

Related open issues

• gh pr view --json comments does not include review comments #11477 — “gh pr view --json comments does not include review comments” (addresses listing inline/review comments)
• Fine Grained Code Review Support #359 — “Fine Grained Code Review Support” (umbrella request around inline/file comments and review operations)
• Output in-line comments summary in gh pr status #9200 — “Output in-line comments summary in gh pr status” (visibility of inline comments)
• Add support for --commit-id on gh pr review #12201 — “Add support for --commit-id on gh pr review” (review tied to specific commit)
• gh pr [comment] not working correctly for few days #8758 — “gh pr [comment] not working correctly for few days” (reliability considerations for comment operations)
• Submit a suggested change to a PR from the Github CLI #5905 — “Submit a suggested change to a PR from the Github CLI” (richer inline interactions)
• Ability to request re-review after addressing requested changes on a PR #2053 — “Ability to request re-review after addressing requested changes on a PR” (review workflow UX)
• gh pr/issue comment allow editing selected comments #10865 — “gh pr/issue comment allow editing selected comments” (deterministic comment manipulation)

Implementation notes

• Reading and replying can leverage REST endpoints.
• True pending-review composition (open → add inline → submit) is best supported via GraphQL mutations.
• Commands should follow gh conventions: standard repo resolution (with -R/--repo override), consistent flags/help, and output options (e.g., --json).

Acceptance criteria

• New commands available under gh pr with docs and examples.
• Unit/integration tests cover happy paths for REST list/reply and GraphQL pending open/add/submit, plus key error cases.
• No collision with existing positional usage (e.g., gh pr review <branch>).

Thank you for considering this proposal.