feat: gate decision-type validation, force-advance, and frontend fixes

Gate (tht-gate.js):
- Pre-validate decision types against workflow.yaml before showing reviewer widget
- Reject decisions emitted by later phases (min-phase check)
- Copy top-level `kind` into artifact when model forgets it (prevents loop)
- Force-advance on reviewer_decide/schema_linking when advance:true — skip
  redundant reviewer_confirm gate

Backend:
- Emit agent_end on clean Pi exit (code 0 + bridge idle) instead of marking failed

Frontend:
- Strip <think> tags from transcript and activity panel
- Fix mermaid render with offscreen container + cleanup
- Graceful mermaid error: show source code instead of red error, fall back to table

Workflow:
- F2 now emits table_promoted and table_excluded (early schema linking decisions)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/viewers/mermaid.ts

@@ -7,6 +7,15 @@ export async function renderMermaid(def: string): Promise<string> {
     _initialized = true;
   }
   const id = `mermaid-${Math.random().toString(36).slice(2)}`;
-  const { svg } = await mermaid.render(id, def);
-  return svg;
+  const container = document.createElement("div");
+  container.style.position = "absolute";
+  container.style.left = "-9999px";
+  document.body.appendChild(container);
+  try {
+    const { svg } = await mermaid.render(id, def, container);
+    return svg;
+  } finally {
+    container.remove();
+    document.getElementById(id)?.remove();
+  }
 }