The OpenAPI document describes a pull comment, a pull variant, a steered run and a prompt run #5114

Intent: a harness can talk to a pull and a running agent through OpenAPI alone.

Done:

• [ ] Rows for POST ${REPO}/pulls/{number}/comments and POST ${REPO}/pulls/{number}/variant (directly after the ${REPO}/pulls/{number}/reviews row), POST ${REPO}/runs/{id}/steer (directly after ${REPO}/runs/{id}/cancel) and POST ${REPO}/prompts/{id}/run (directly after ${REPO}/prompts/{id})
• [ ] Each with auth, the body fields its handler reads, success status when not 200, and refusals by stable error code; summaries say who may call it, as the handler checks it
• [ ] test/openapi.test.ts still passes, and a new test asserts each new path and method is in the served document

Files: src/contract/openapi.ts, one test. Change no other row.