[TASK] Integrate C8Run into Release Train BPMN

[bkenez]

Related to

Epic: https://github.com/camunda/team-distribution/issues/693
Depends on: C8Run GHA workflows (#319)

Flow

The release-train BPMN drives the C8Run GHA workflows via the C8/GitHub connector
(POST /repos/camunda/camunda/actions/workflows/{file}/dispatches), exactly as it does for the Helm
charts. The connector only dispatches and reads run status — the workflow does all building and
uploading.

RC

1. GitHub connector → c8run-release-rc.yaml with {branch, camundaVersion, connectorsVersion}.
   (The Docker-Compose-readiness check is internal to this workflow — no separate preflight step.)
2. On failure → Activity_1yhi0yt userTask (fallback).

Public (after QA sign-off)

1. GitHub connector → c8run-release-public.yaml with {rc-tag}.
2. On failure → Activity_0gayhob userTask (fallback).

QA

Already automated via Activity_1cnjz96 — no changes.

Notes

• Build versions are passed in the dispatch inputs (the C8Run analog of Helm's image-tag inputs);
  naming, minor version, and prerelease flag are derived inside the workflows, keeping the dispatch
  payloads minimal.
• Artifacts URL deterministic: https://downloads.camunda.cloud/release/camunda/c8run/{version}/
  (unchanged from today).
• No cleanup step — build-once → promote leaves no -rc artifacts on the public release to delete.