v6.0.0 — base-build-scratch promote auto-gates on build_change_paths

Removes the promote_job_rules input; promote shares the build jobs' change-path
gate (heavy-job-rules minus merge_request) via an internal .promote-rules anchor.
A docs/CI-only push no longer runs promote or fires a cascade, and build and
promote can't drift. BREAKING: consumers passing promote_job_rules must drop it.
Backfills v5.0.0/v5.0.1 CHANGELOG entries.