Resolves #3608884: Declare run scope and fix misleading _meta wording

Scope key in BOTH llm and json (widened from the summary's llm-only — CI scripts consuming json share the blind spot); _meta rewording; exit_codes object cast (json_encode was silently degrading the keyed map to a positional array — the summary's item 3 turned out to be a serialization bug, not a design choice); envelope schema v2. See the issue summary.