The preflight half of #188, landed first so it stands alone: the forge is
decided once, before any sweep, and a client that cannot speak it exits
non-zero with a named reason.
Measured against forgejo.heavyduty.builders at 84bb1a4 — two of the three
actions reported SUCCESS having read nothing:
labels-scope exit 0 "no .github/labeler.yml" (the file is HTTP 200)
labels-reconcile exit 0 "reconciled." (zero PRs enumerated)
issueflow-reconcile exit 1 "unexpected end of JSON input"
labels-reconcile's blind-sweep warning (#96) could not fire: it counts
unreadable PRs against a list `gh pr list` never produced, and a process
substitution's failure does not trip set -e, so total stayed 0. Installing
gh makes it worse, silencing the one loud failure.
Detection is measured, not inferred from docs: a real forgejo-runner v6.3.1
job (probe task 278) shows Forgejo populating the whole GITHUB_* namespace,
so GITHUB_ACTIONS proves nothing. GITHUB_API_URL's shape, GITEA_ACTIONS and
GITHUB_SERVER_URL do. The same probe shows the runner image carries neither
gh nor stoke, which is what makes the forgejo backend REST.
Tests declare CEREMONY_FORGE at the forge boundary rather than stubbing gh
and staying silent about the forge — the boundary move term 5 asks for.
Refs #188
cluade-reviewer-andresmgsl
7d52b2cd4a