feat(model-routing): blocking model-gate include (self-check + witness)

lib/model-gate.md

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+# Model gate — reflection requires a big model (BLOCKING)
+
+Shared include. Runs FIRST in any orchestrator whose reflection —
+brainstorming, planning, contract, audit judgment, loop decisions —
+executes inline or in inherit-model subagents. Sonnet-pinned executors are
+not what this gate protects; it protects the thinking around them (BDR-066).
+
+## 1. Self-check
+
+Your system prompt names the model powering this session. Fable or Opus →
+big. Sonnet, Haiku, anything else → small.
+
+## 2. Witness — deterministic check
+
+    bash "$HOME/.claude/lib/model-check.sh"
+
+Output `<class>:<raw>`; exit 0 = big, 2 = small, 3 = unknown. The witness
+reads the PERSISTED model (settings.json — the file `/model` rewrites,
+LRN-098). It can lag reality (session launched with `--model`, settings not
+yet rewritten) — that is why the self-check exists alongside it.
+
+## 3. Verdict
+
+| self-check | witness | action |
+|---|---|---|
+| big | big (0) | proceed, SILENT — the nominal path prints nothing |
+| small | any | **STOP** |
+| big | small (2) | disagreement — **STOP**, surface BOTH values; the user confirms or relaunches |
+| big | unknown (3) | fail-visible: print `model gate: witness unknown (<raw>) — self-check says <model>` and ask the user to confirm before continuing (BDR-025: unknown never silently passes) |
+
+**STOP means**: print exactly
+
+    ⛔ MODEL GATE — session on <model>. Reflection steps of this skill
+    require Fable or Opus. Switch with /model, then relaunch the skill.
+
+then end the turn. No later step runs, no agent is dispatched, nothing is
+edited.