feat(implement-loop): tier the quality review by independently-measured diff size

qual+repairs are the pipeline's largest cost pool (51M of 164.5M real
tokens, 31%, in the 206-run corpus), with opus weighing ~5x on the
quota side. The opus pin's own rationale (consequence-of-a-miss,
agent-template § model) scales with diff size — so the tier now does
too: the spec-reviewer, which runs before quality, reads the diff
itself and is independent of the implementer, reports diff_magnitude
(git diff HEAD --shortstat) and touches_contract; <=25 changed lines
and no contract-referenced path -> sonnet/high, anything larger,
contract-touching, or unmeasured -> opus/xhigh (fail conservative).
The independent end-verify/mini-verify suite gate is the
deterministic backstop that makes the lower tier defensible; the
chosen tier is logged per task (qual_tier) in the end-report.

Sampling phase per the issue's acceptance: the first ~10 real
small-diff iterations get a one-off opus second review; important+
divergences are recorded on the issue before the threshold counts as
proven.

closes #30

implement/agents/quality-reviewer.md

@@ -19,6 +19,14 @@ has reported `compliant`, never before.
 only after the spec-compliance phase reports `compliant` —
 a separate `agent()` call with a fresh context.)
 
+The frontmatter pins opus/xhigh as this role's default tier;
+the workflow overrides it per dispatch (issue #30): a small
+(≤25 changed lines), non-contract diff — as measured
+independently by the spec-reviewer, never by the implementer —
+runs at sonnet/high, with the loop's independent end-verify
+suite gate as the deterministic correctness backstop. The
+review bar and this file's rules are identical at both tiers.
+
 ## What this role is for
 
 Spec compliance answers "did the implementer build the right