fix(process_registry): kill orphaned Popen on post-spawn setup failure

After Popen succeeds with os.setsid (detached process group), 5 things
happen with no try/except: Thread construction, reader.start(), lock
acquisition, prune+register, checkpoint write. If any raises, the
Popen object goes unregistered and the detached process group leaks
indefinitely.

Wrap the post-spawn setup in try/except. On failure:
  - os.killpg(getpgid(pid), SIGKILL) takes down the entire process
    group (not just the shell - important because of detached PG +
    -lic shell wrapper that may have spawned children)
  - proc.kill() fallback for ProcessLookupError/PermissionError/OSError
  - proc.wait(timeout=5) reaps with a bound
  - re-raise to preserve original traceback
Nested try/except around cleanup so a secondary failure can't mask the
original.

Closes #2749.

tools/process_registry.py

@@ -562,21 +562,42 @@ class ProcessRegistry:
         session.process = proc
         session.pid = proc.pid
 
-        # Start output reader thread
-        reader = threading.Thread(
-            target=self._reader_loop,
-            args=(session,),
-            daemon=True,
-            name=f"proc-reader-{session.id}",
-        )
-        session._reader_thread = reader
-        reader.start()
+        try:
+            # Start output reader thread
+            reader = threading.Thread(
+                target=self._reader_loop,
+                args=(session,),
+                daemon=True,
+                name=f"proc-reader-{session.id}",
+            )
+            session._reader_thread = reader
+            reader.start()
 
-        with self._lock:
-            self._prune_if_needed()
-            self._running[session.id] = session
+            with self._lock:
+                self._prune_if_needed()
+                self._running[session.id] = session
+
+            self._write_checkpoint()
+        except Exception:
+            # Post-Popen setup failed — kill the orphaned subprocess (and any
+            # descendants spawned via setsid) before re-raising so they do not
+            # leak as untracked background processes.
+            try:
+                if not _IS_WINDOWS:
+                    try:
+                        os.killpg(os.getpgid(proc.pid), signal.SIGKILL)
+                    except (ProcessLookupError, PermissionError, OSError):
+                        proc.kill()
+                else:
+                    proc.kill()
+            except Exception:
+                pass
+            try:
+                proc.wait(timeout=5)
+            except Exception:
+                pass
+            raise
 
-        self._write_checkpoint()
         return session
 
     def spawn_via_env(