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

[SamMorrowDrums]

Nicely scoped fix. The splitPullRequestReviewers helper is the right shape, tests cover the user+team mixed case, and the schema descriptions are updated to match.

I rebased onto current main and validated locally:

• script/lint — 0 issues
• go test ./pkg/github -run 'Test_UpdatePullRequest|TestGranularRequestPullRequestReviewers' -count=1 — pass
• script/test — pass

Edge cases I poked at all behave sensibly:

• "user" → user reviewer ✅
• "owner/team" → team_reviewers=["team"] ✅
• "/team", "owner/", "a/b/c" → fall through to user reviewers (GitHub will reject; minor footgun but matches GH CLI's --reviewer syntax)

The org prefix is dropped at the API boundary; if the org doesn't match the repo's org, GitHub will return an error rather than us pre-validating — that's fine. Approving once CI is green.

Thanks @he-yufeng!