Measured while commenting on PR 102: POST /pulls/102/comments answers 405 to a
plain body (it is the review-comment store); the conversation thread is
POST /issues/<PR-number>/comments, which answers 201 and returns an html_url on
the PR. Gitea maintains one issue index, so the PR number is the same number.
The trap that hid it: building the body with jq --rawfile b /dev/stdin inside a
command substitution feeds jq nothing (the substitution does not inherit the
heredoc), Gitea answers 422, and jq -r .id on the error prints nothing — a
silent no-op that looked like a posted comment. The body goes in a file.