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/MarkdownView.tsx

@@ -16,7 +16,7 @@ function MermaidBlock({ code }: { code: string }) {
       );
   }, [code]);
 
-  if (error) return <pre className="text-destructive text-xs">{error}</pre>;
+  if (error) return <pre className="overflow-auto rounded-lg bg-muted p-3 text-xs opacity-60">{code}</pre>;
   if (!svg) return <pre className="text-xs opacity-50">{code}</pre>;
   // eslint-disable-next-line react/no-danger
   return <div dangerouslySetInnerHTML={{ __html: svg }} />;