ci: add runner-only publish-local workflow for dead-RE fallback

The remote execution backend (cache.projectbluefin.io) is down and
build.yml hard-codes RE for the full build, leaving :testing stale for
12 days. publish-local.yml builds the entire graph on GitHub-hosted
runners with RE off, carrying the BuildStream cache across seed shards
and sequential link jobs via workflow artifacts, then exports, lints,
boot-checks, pushes :SHA and promotes :testing. Re-dispatch resumes
from the newest surviving cache artifact.

Assisted-by: Claude Fable 5 via GitHub Copilot

Author: castrojo