Verify: Fase 1 triage pipeline (docs-only)

Mohamed Arequested to merge
verify/fase1-triage into main

Docs-only verification MR. Goal: prove reachgate-triage runs on merge_request_event, posts both receipt comments (A REACHABLE / B NOT_REACHABLE), renders certificates, and uploads reachgate-receipts.json as artifact. No feature scope.

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