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>

backend/src/pi/pi-process-manager.ts

@@ -122,6 +122,14 @@ export class PiProcessManager {
       child.on("exit", (code) => {
         console.error(`[pi:${sessionId}] exited code=${code ?? "?"} mapped=${this.runtimes.get(sessionId) === runtime}`);
         if (this.runtimes.get(sessionId) === runtime) {
+          // Clean exit (code 0) while the bridge is idle means the model completed its
+          // turn and Pi shut down normally (e.g. after session finalization). Just clean
+          // up — no failure events.
+          if (code === 0 && runtime.bridge.turnState() === "idle") {
+            this.runtimes.delete(sessionId);
+            runtime.bridge.emitClientEvent({ type: "system_event", event: "agent_end" });
+            return;
+          }
           runtime.bridge.markFailed();
           this.runtimes.delete(sessionId);
           runtime.bridge.emitClientEvent({