The OpenAPI document describes a repository's code writes: create and delete a branch, write a file

Intent: a harness can change a repository's code through OpenAPI alone.

Done:

[ ] src/contract/openapi.ts has rows for POST ${REPO}/branches, DELETE ${REPO}/branches/{branch} and POST ${REPO}/contents, each with auth, the body fields its handler in src/http/api.ts reads, its success status when not 200, and the refusals it answers by stable error code; each summary says the answer's shape as the handler answers it today
[ ] Placed as one block directly after the ${REPO}/edits row, changing no other row (other OpenAPI issues insert at other rows in parallel)
[ ] 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. Do not change any route.

The OpenAPI document describes a repository's code writes: create and delete a branch, write a file #5112