Problem Statement

When reviewing PRs, I often need to resolve/unresolve review threads. The gh CLI currently has no command for this, so I have to fall back to gh api graphql to call resolveReviewThread manually.

Proposed Solution

Add a first‑class command to resolve/unresolve review threads, e.g.

• gh pr review-thread resolve <thread-id>
• or gh pr review --resolve-thread <thread-id>
• optionally: gh pr review-thread list --unresolved

Example Use Case

In CI workflows or scripted review tooling, I want to automatically resolve a thread after pushing a fix without leaving the CLI. Today this requires GraphQL API calls.

Additional Context

There is existing demand for finer‑grained PR review actions (see #359). This request is specifically for “resolve conversation” on review threads.