fix: support team pull request reviewers

[he-yufeng]

Summary

• allow reviewers to include ORG/team-slug entries for pull request review requests
• split team reviewer entries into GitHub's team_reviewers request field while keeping ordinary usernames in reviewers
• apply the same behavior to the consolidated update_pull_request tool and the granular request_pull_request_reviewers tool
• update the tool schema snapshot and README wording to document the accepted format

To verify

• go test ./pkg/github -run 'Test_UpdatePullRequest|TestGranularRequestPullRequestReviewers'
• go test ./pkg/github
• git diff --check

Fixes #2440