gh api review replies/reviewers: form strings 422 — use JSON body

gh api review endpoints reject form-encoded strings: gh api -f in_reply_to=3730403846 -f commit_id=... returns HTTP 422 (oneOf subschema) because -f sends strings and in_reply_to must be an INTEGER; positioning (commit_id/path/line, or position) is also required. Fix: POST a raw JSON body via -d/--input, passing in_reply_to as a number plus valid positioning. Same class of bug: POST /pulls/{n}/review-requests requires reviewers as a REAL JSON array ({"reviewers": ["BYK"]}) — a JSON-encoded string "[\"BYK\"]" also 422s. Both hit during PR #865 review cleanup.