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/SchemaLinkingViewer.tsx

@@ -116,9 +116,9 @@ export function SchemaLinkingViewer({ linking }: { linking: SchemaLinking }) {
     if (view !== "chart" || oversized) return;
     let cancelled = false;
     const def = buildErDiagram(promoted, linking.joins);
-    renderMermaid(def).then((s) => {
-      if (!cancelled) setSvg(s);
-    });
+    renderMermaid(def)
+      .then((s) => { if (!cancelled) setSvg(s); })
+      .catch(() => { if (!cancelled) setView("table"); });
     return () => {
       cancelled = true;
     };