feat: explicit effort-tier model across the design-phase spine

Add a Trivial/Small/Standard tier model so the design ceremony scales to
the change while the evidence gate never does. README gains a canonical
'Sizing the work' table + the non-negotiable rule (verification-gate and
code-review-loop always run); shape-spec gets a Step 0 sizing gate;
write-plan and plan-review state which tier they belong to.

skills/plan-review/SKILL.md

@@ -35,12 +35,18 @@ between `write-plan` and implementation.
 
 ## When NOT to Use
 
-- The plan is for a single-file, single-author change (use code review instead)
+- The change is **Trivial** or **Small** (see README "Sizing the work") — plan-review is a Standard-tier step; smaller changes go straight to implementation and `code-review-loop`
 - A previous plan-review already passed and the plan hasn't changed since
 - You don't have a written plan yet (use `write-plan` first)
 
 ## Process
 
+**Effort tier:** plan-review is a **Standard**-tier gate — it earns its cost when a
+plan carries real design decisions across multiple files. **Trivial** and **Small**
+changes skip it; their safety net is `verification-gate` plus `code-review-loop` on
+the diff, which never skip regardless of tier. Reviewing a one-task plan is the kind
+of ceremony that teaches people to route around the gates that matter.
+
 ### Step 1: Locate and read the plan
 
 **Goal:** Confirm the plan file exists and meets the minimum bar to be reviewed.