fix(bridge): forward tool/lifecycle events so user sees agent progress

The SessionBridge only forwarded text_delta and system_event types.
All tool_execution_*, agent_start, and turn_end events from the Pi
process were silently dropped, so the user saw a blank session even
though the agent was actively running (calling tools, querying the DB).

Forward:
- tool_execution_start/end as info events (visible in stepMessages)
- agent_start, turn_end as system_event (lifecycle tracking)

Also: log Pi stderr instead of draining silently, for debugging.

backend/src/bridge/session-bridge.ts

@@ -18,29 +18,31 @@ export class SessionBridge {
   constructor(private rpc: RpcClient) {
     rpc.on("event", (m) => {
       if (m.type === "extension_ui_request" && m.method === "input") {
-        let descriptor: any;
-        try { descriptor = JSON.parse(m.title); } catch { return; }
+        let descriptor: unknown;
+        try { descriptor = JSON.parse(m.title as string); } catch { return; }
         this.pending = descriptor;
-        this.pendingPiId = m.id;
+        this.pendingPiId = m.id as string | null;
         this.fan({ type: "ui_request", ui_request: descriptor });
       } else if (m.type === "extension_ui_request" && m.method === "notify") {
         this.fan({ type: "info", level: m.notifyType ?? "info", text: m.message ?? "" });
       } else if (m.type === "message_update" && m.assistantMessageEvent?.type === "text_delta") {
-        // Real Pi streams assistant text as message_update events whose nested
-        // assistantMessageEvent carries the incremental `delta`. Map it to the FE's
-        // text_delta contract. (Top-level text_delta below is what the fake-pi-rpc emits.)
         this.fan({ type: "text_delta", text: m.assistantMessageEvent.delta ?? "" });
       } else if (m.type === "text_delta") {
         this.fan({ type: "text_delta", text: m.text ?? "" });
+      } else if (m.type === "tool_execution_start") {
+        this.fan({ type: "info", level: "info", text: `▶ ${m.toolName ?? "tool"}: ${JSON.stringify(m.args ?? {}).slice(0, 200)}` });
+      } else if (m.type === "tool_execution_end") {
+        const summary = m.result?.content?.[0]?.text?.slice(0, 200) ?? "";
+        this.fan({ type: "info", level: "info", text: summary ? `✓ ${m.toolName ?? "tool"}: ${summary}` : `✓ ${m.toolName ?? "tool"}` });
       } else if (m.type === "system_event") {
         this.fan(m as ClientEvent);
       } else if (m.type === "agent_end") {
-        // Fine turno di Pi (pi-agent-core agent-loop): e' l'unico segnale che il turno e'
-        // concluso. Senza inoltrarlo, il FE resta "working" per sempre quando il turno
-        // finisce senza un gate successivo (ultimo step del workflow).
         this.fan({ type: "system_event", event: "agent_end" });
+      } else if (m.type === "agent_start") {
+        this.fan({ type: "system_event", event: "agent_start" });
+      } else if (m.type === "turn_end") {
+        this.fan({ type: "system_event", event: "turn_end" });
       }
-      // altri method nativi (setStatus/setWidget) e altri eventi Pi ignorati in MVP
     });
   }