Promotion is a generic, hands-free primitive

The testing to master merge becomes as hands-free, forgiving, smart, resilient and agnostic as possible, in the repository and generic: a repeatable promotion primitive (any source ref to any target ref through a declared projection and boundary, idempotent, runnable any number of times, usable for other pairs such as the docs-lab graduation or other harness repositories). No one-off scripts, no hard-coded branch names or paths: all configuration. Its tasks land before the first promotion after the first committed testing.