ci(ai-validation): clarify where schema-mismatch protection lives (Copilot)

Copilot finding on this PR: the previous comment block claimed the
workflow's fail-closed gate catches DB schema incompatibilities, but
that gate is purely a presence check (`-d .reference-db/extracted`)
— it does not load or validate the DB. The actual schema-mismatch
protection lives inside the pinned reviewer Python code (loaded via
`uv pip install ./reviewer-src` from `env.REVIEWER_REF`): newer
reviewer code would error at DB-load time inside the `Pass 1 —
deterministic checks` step.

Reword the rationale so it correctly attributes the protection to
where it actually executes. Comment-only change; no behavior delta.

Will sync the same wording to rolling/circinus/canonical in a small
follow-up batch once this PR merges.

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