fix(ci): update GitLab CI configuration to include workflow rules for pipeline sources

Refines the GitLab CI pipeline configuration to control job execution based on the pipeline source, avoiding unnecessary runs and separating scheduled from event-driven pipelines.

Changes

• Added workflow.rules to restrict pipeline creation to three sources: schedule, push, and merge_request_event.
• Scoped the include of osm-base.yml (from devops/cicd) exclusively to scheduled pipelines, preventing it from running on every push or MR.
• Added a lightweight build_job for push and merge_request_event sources that redirects developers to check build progress on Jenkins.

Motivation

The previous configuration ran the full OSM base template on all pipeline triggers. This change ensures:

• Scheduled pipelines (e.g., nightly jobs) continue using the full base template.
• Push and MR events only trigger a minimal job, since the actual build is handled by Jenkins.

Testing

• Verified pipeline behavior for push, merge_request_event, and schedule trigger sources.