Add GitHub PR review integration

[jasdumas]

Problem

counselor only works at commit time via pre-commit hooks. Many teams review code at the PR level, and AI-generated PRs from Copilot/Claude need the same human oversight.

Proposed Solution

• Add review_pr() function that fetches a GitHub PR diff and starts a voice review
• Support for:
  • review_pr(pr_number) — review a specific PR
  • review_pr(url) — review from a PR URL
  • GitHub Actions integration: trigger voice review on PR creation
• Post review outcome as PR comment (approved/changes requested)
• Use `gh` CLI or `gh` R package for GitHub API access

Example

# Review PR #42
counselor::review_pr(42)

# Review from URL
counselor::review_pr("https://github.com/org/repo/pull/42")

Acceptance Criteria

• Can review any GitHub PR via voice
• Review outcome posted as PR comment
• Works with both public and private repos