commit statuses API and MR check display #1

External CI can already react via webhooks, but there is no way to report a result back: no commit-status endpoint, nothing on the MR page showing checks, and merges cannot gate on them. Unlocks the whole CI story. Suggested shape: a status command/API keyed on (repo, sha, context, state), shown on MR and commit pages, with an optional require-checks branch setting.

Shipped: status set/list (upsert per context, worst-of combined), write-access requirement, webhook status events, display on commit/MR pages and mr show, and repo settings require-checks merge gating with named offending contexts. Approvals and CODEOWNERS remain #19.