feat(ci): add unified "OCI: Build, Compute image, Test, Publish" workflow

Single workflow_dispatch that builds the qcow2 images, creates a Compute
Custom Image from each one in-job on the build runner (Object Storage
upload + import + capability schema + shape compatibility, no S3
round-trip), boot-tests every Compute Image, and publishes each image
that passed to the Oracle Cloud Marketplace as a draft revision.

Stage gating:
  create_compute_image=false   -> build only
  release_to_marketplace=false -> build + compute-image + test

Test runs for every Compute Image; publishing covers exactly the images
that passed, and publish legs run in parallel (one listing per arch).

Stages are factored into composite actions extracted from the standalone
oci-test.yml / oci-marketplace-publish.yml, with secrets/vars passed as
inputs and job.status replaced by step outcomes so they work inside a
composite:
  oci-compute-image-steps - Object Storage upload + Compute Image import
  oci-test-steps          - Compute Image boot test
  oci-marketplace-steps   - Marketplace draft publish

Per-image data flows between matrix stages through manifest artifacts.

Author: yuravk