Opt-in: with the env set, loading a legacy omnigraph.yaml is a hard
error pointing at config migrate — the regression guard for migrated
teams (a stray legacy file would otherwise silently outrank operator
config during the window) and the rehearsal for stage 5's removal.
Strict refuses the FILE, never its absence: flag-less invocations on
migrated setups are untouched. Inert unless set.
The RFC's stages-1-3-then-4 release gap collapsed honestly: no version
boundary was crossed between them, so all four ship in the same release
(noted in the RFC).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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