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/pi/pi-process-manager.ts

@@ -59,8 +59,8 @@ export class PiProcessManager {
       cwd: this.cfg.harnessDir,
       env,
     });
-    // Drain stderr so the child's stderr buffer never blocks the process.
-    child.stderr.resume?.();
+    // Log stderr for debugging (was silently drained)
+    child.stderr.on("data", (d: Buffer) => console.error(`[pi:${sessionId}] stderr:`, d.toString().trim()));
     return child;
   }
 
@@ -95,6 +95,7 @@ export class PiProcessManager {
     // exit event fires, so reaching this branch with `rt` still mapped means the child
     // died on its own: tell the client, or the UI spins forever waiting for a turn end.
     child.on("exit", (code) => {
+      console.error(`[pi:${sessionId}] exited code=${code ?? "?"} mapped=${this.runtimes.get(sessionId) === rt}`);
       if (this.runtimes.get(sessionId) === rt) {
         this.runtimes.delete(sessionId);
         rt.bridge.emitClientEvent({