Add Goblin control primitive (docs + minimal runtime helper)

docs/goblin-control-primitive.md

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+# Goblin as Control Primitive
+
+This document operationalizes **Goblin** as a control primitive for agents and systems.
+
+## Definition
+
+Goblin = system state is not trustworthy for decision-making.
+
+## Required Behavior
+
+When Goblin is detected:
+
+- Do not trust current state
+- Gate irreversible actions
+- Capture cross-plane state
+- Surface explicitly
+
+## Trigger Conditions
+
+- Cross-plane inconsistency
+- Presented vs actual divergence
+- Orphaned authority
+- Unresolvable truth source
+
+## Agent Rule
+
+```
+IF inconsistency_detected:
+  emit GoblinEvent
+  set severity >= 2
+  block irreversible actions
+```
+
+## Notes
+
+Goblin is the front-door signal. Root cause (model gap, drift, SCIG) is secondary.

src/goblin/control.py

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+def detect_goblin(inconsistency: bool, severity: int = 2):
+    if not inconsistency:
+        return None
+
+    return {
+        "type": "GoblinEvent",
+        "severity": max(severity, 2),
+        "action": "block" if severity >= 3 else "gate"
+    }