fix(collector): daemonize background subprocesses with start_new_session

Collector and mutator watcher subprocesses were spawned without
start_new_session=True, leaving them in the parent's process group.
SIGHUP (sent when the controlling terminal closes) killed both
processes silently — stdout/stderr were DEVNULL so the crash was
invisible.

Also update test_services and test_composer to reflect the ssh plugin
no longer using Cowrie env vars (replaced with SSH_ROOT_PASSWORD /
SSH_HOSTNAME matching the real_ssh plugin).

decnet/cli.py

@@ -390,7 +390,8 @@ def deploy(
             subprocess.Popen(  # nosec B603
                 [sys.executable, "-m", "decnet.cli", "mutate", "--watch"],
                 stdout=subprocess.DEVNULL,
-                stderr=subprocess.STDOUT
+                stderr=subprocess.STDOUT,
+                start_new_session=True,
             )
         except (FileNotFoundError, subprocess.SubprocessError):
             console.print("[red]Failed to start mutator watcher.[/]")
@@ -405,6 +406,7 @@ def deploy(
             [sys.executable, "-m", "decnet.cli", "collect", "--log-file", str(effective_log_file)],
             stdout=subprocess.DEVNULL,
             stderr=subprocess.STDOUT,
+            start_new_session=True,
         )
 
     if api and not dry_run: