The mid-tier model directive lived only in a prose "Model tiering"
rationale subsection, separated from the "Spawning" subsection that
governs actual dispatch. Under multi-agent parallel dispatch (12
reviewers), the upstream rationale paragraph reliably fell out of
attention and reviewers spawned on the session model (often Opus)
instead of the platform's mid-tier model — silently multiplying the
cost of a review.

Restate the imperative at the Spawning point-of-action alongside the
existing `mode` paragraph, naming the three session-model exception
reviewers (ce-correctness-reviewer, ce-security-reviewer,
ce-adversarial-reviewer) inline so the agent does not have to recall
them from upstream prose. Cover all three cross-platform dispatch
primitives (Agent / spawn_agent / subagent) and preserve the existing
"omit if unknown" fallback. Add a contract test that extracts the
Spawning subsection and asserts the imperative, platform primitives,
and exception reviewers appear within it, so the directive cannot
drift back out to rationale.

Fixes #679