feat(agents): add Gemini intent gate enforcement overlay for Sisyphus

Counter Gemini's tendency to skip Phase 0 intent classification by
injecting a mandatory self-check gate before tool calls. Includes
intent type classification, anti-skip mechanism, and common mistake
table showing wrong vs correct behavior per intent type.

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/agents/sisyphus.ts

@@ -5,6 +5,7 @@ import {
   buildGeminiToolMandate,
   buildGeminiDelegationOverride,
   buildGeminiVerificationOverride,
+  buildGeminiIntentGateEnforcement,
 } from "./sisyphus-gemini-overlays";
 
 const MODE: AgentMode = "primary";
@@ -567,7 +568,7 @@ export function createSisyphusAgent(
   if (isGeminiModel(model)) {
     prompt = prompt.replace(
       "</intent_verbalization>",
-      `</intent_verbalization>\n\n${buildGeminiToolMandate()}`
+      `</intent_verbalization>\n\n${buildGeminiIntentGateEnforcement()}\n\n${buildGeminiToolMandate()}`
     );
     prompt += "\n" + buildGeminiDelegationOverride();
     prompt += "\n" + buildGeminiVerificationOverride();