Adds the net-new GitHub primitives the PR nodes need: replyToReviewThread and resolveReviewThread (GraphQL mutations, dual gh/API), an ETag-conditional probePrChanged (304 is rate-limit-free, gates the reconcile deep-fetch), and expectedHeadOid on mergePr (--match-head-commit / REST sha) raising a typed PrStaleHeadError on a head-moved race so pr-merge can re-evaluate. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>