feat(routing): W1 no-inherit — fable skill-runners, opus review dispatches, dispatch-tier doctrine (BDR-077)

No dispatched agent inherits the session model anymore:
- client-handover-writer's 7 general-purpose skill-runner dispatch sites
  carry model: "fable" (+ normative rule; spike-verified alias — resolves
  claude-fable-5, enum-validated, loud failure, never silent fallback)
- ship-feature STEP 6 + init-project STEP 10 code-review dispatches carry
  model: "opus" (was: inherit — the leak the maps exposed)
- model-gate.md §4: dispatch-tier doctrine (typed = frontmatter pin,
  built-ins = explicit model= at every call site)
- census §12 (66 pass), make test green

lib/model-gate.md

@@ -35,3 +35,13 @@ yet rewritten) — that is why the self-check exists alongside it.
 
 then end the turn. No later step runs, no agent is dispatched, nothing is
 edited.
+
+## 4. Dispatch tiers (BDR-077 — no inherit)
+
+The gate guards the MAIN loop only. Dispatched work NEVER inherits the
+session model: typed agents run on their frontmatter pin; built-ins
+(general-purpose / Explore / Plan) carry an explicit `model=` at every call
+site — `model: "fable"` when the child performs reflection/orchestration on
+the main loop's behalf (skill-runners), otherwise its complexity tier
+(opus = dispatched judgment, sonnet = execution/collection, haiku = short
+mechanical probes).