GitLab commit status + Bitbucket build status reporting

Context

The webhook input trio is closed (#11 GitLab, #12 Bitbucket): all three providers trigger pipelines through the same dispatch path. But the output side is asymmetric — only GitHub gets run feedback, via the GitHub App check-run reporter (store/checks.go). GitLab MRs and Bitbucket PRs trigger runs whose result never shows up on the MR/PR page.

Proposal

Mirror the check-run reporter for the other two providers:

• GitLab: POST /projects/:id/statuses/:sha (commit status API) — states pending/running/success/failed/canceled, target_url pointing at the run detail page.
• Bitbucket Cloud: POST /2.0/repositories/{ws}/{repo}/commit/{sha}/statuses/build — states INPROGRESS/SUCCESSFUL/FAILED/STOPPED.

Same lifecycle hooks as the GitHub reporter: status created on run creation, updated on completion. Auth via the scm_source credentials already stored for webhook registration.

Why it matters

Without this, on: [pull_request] works but reviewers can't see CI state on the MR/PR — they have to open gocdnext manually. Table stakes in GitLab CI / GitHub Actions and a prerequisite for the pr-comment plugin and any merge-gating workflow.

Start with store/checks.go and trace the existing GitHub check-run reporter’s run-creation and completion hooks. Locate the scm_source credential path used for webhook registration, then map the GitLab and Bitbucket status API calls to those hooks. Done means both providers show pending or running status and update it with the completed result and run URL.